GC improvements 6: introduce batched GC

[teh-cmc]

Makes the GC capable of dropping entire buckets in one go when the conditions are met (and they are pretty simple to meet in the common case of in-order data).

Unfortunately, I couldn't make the batched GC match -- let alone improve -- the performance of the standard GC.
I even have a branch with a parallel batched GC, and it's still slower: the overhead of the batching datastructures just kills me everytime.
For that reason, batching is disabled by default.

I still want to commit the code so as to prevent it from rotting though, so we can come back to it at a later time.
This introduces a slight performance deterioration on the non-batched path, that's fine.

Benchmarks

Compared to main:

group                                                                       gc_improvements_0                       gc_improvements_6
-----                                                                       -----------------                       -----------------
.../plotting_dashboard/drop_at_least=0.3/default                            7.62    652.8±4.12ms 89.8 KElem/sec     1.00     85.7±1.14ms 683.8 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=256                       5.34    465.8±2.50ms 125.8 KElem/sec    1.00     87.2±0.55ms 671.9 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=512                       7.12    655.3±2.61ms 89.4 KElem/sec     1.00     92.0±1.85ms 636.8 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=1024                      12.45  1084.0±4.47ms 54.1 KElem/sec     1.00     87.1±0.40ms 672.7 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=2048                      23.63      2.1±0.02s 27.6 KElem/sec     1.00     89.9±1.13ms 652.0 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=256/gc_batching=true                                              1.00     98.6±0.82ms 594.2 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=512/gc_batching=true                                              1.00     94.6±1.26ms 619.3 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=1024/gc_batching=true                                             1.00     93.2±1.29ms 628.9 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=2048/gc_batching=true                                             1.00     94.3±1.43ms 621.1 KElem/sec
.../timeless_logs/drop_at_least=0.3/default                                 33.30      2.4±0.03s 24.4 KElem/sec     1.00     72.2±2.46ms 811.4 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=256                            35.16      2.5±0.08s 23.5 KElem/sec     1.00     71.1±2.31ms 824.5 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=512                            35.08      2.4±0.02s 24.5 KElem/sec     1.00     68.1±1.20ms 859.9 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=1024                           36.86      2.4±0.05s 24.2 KElem/sec     1.00     65.7±0.87ms 891.4 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=2048                           35.99      2.4±0.03s 24.1 KElem/sec     1.00     67.7±1.33ms 865.9 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=256/gc_batching=true                                                   1.00     68.7±1.40ms 853.1 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=512/gc_batching=true                                                   1.00     67.3±0.32ms 870.8 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=1024/gc_batching=true                                                  1.00     67.7±1.21ms 865.2 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=2048/gc_batching=true                                                  1.00     67.6±1.31ms 866.6 KElem/sec

Compared to previous PR:

group                                                                       gc_improvements_5                       gc_improvements_6
-----                                                                       -----------------                       -----------------
.../plotting_dashboard/drop_at_least=0.3/default                            1.00     81.4±0.94ms 720.0 KElem/sec    1.05     85.7±1.14ms 683.8 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=256                       1.00     84.0±0.50ms 697.8 KElem/sec    1.04     87.2±0.55ms 671.9 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=512                       1.00     82.5±1.33ms 710.0 KElem/sec    1.11     92.0±1.85ms 636.8 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=1024                      1.00     83.4±1.16ms 702.9 KElem/sec    1.04     87.1±0.40ms 672.7 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=2048                      1.00     83.7±0.61ms 700.0 KElem/sec    1.07     89.9±1.13ms 652.0 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=256/gc_batching=true                                              1.00     98.6±0.82ms 594.2 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=512/gc_batching=true                                              1.00     94.6±1.26ms 619.3 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=1024/gc_batching=true                                             1.00     93.2±1.29ms 628.9 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=2048/gc_batching=true                                             1.00     94.3±1.43ms 621.1 KElem/sec
.../timeless_logs/drop_at_least=0.3/default                                 1.00     66.8±0.85ms 877.3 KElem/sec    1.08     72.2±2.46ms 811.4 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=256                            1.00     67.5±1.43ms 868.2 KElem/sec    1.05     71.1±2.31ms 824.5 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=512                            1.00     67.4±1.40ms 869.4 KElem/sec    1.01     68.1±1.20ms 859.9 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=1024                           1.03     67.5±2.21ms 867.5 KElem/sec    1.00     65.7±0.87ms 891.4 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=2048                           1.00     67.8±1.86ms 863.9 KElem/sec    1.00     67.7±1.33ms 865.9 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=1024/gc_batching=true                                                  1.00     67.7±1.21ms 865.2 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=2048/gc_batching=true                                                  1.00     67.6±1.31ms 866.6 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=256/gc_batching=true                                                   1.00     68.7±1.40ms 853.1 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=512/gc_batching=true                                                   1.00     67.3±0.32ms 870.8 KElem/sec

---

Part of the GC improvements series:

• GC improvements 0: use-case driven benchmarks #4394
• GC improvements 1: fix timeless tables not being RowId-ordered #4395
• GC improvements 2: VecDeque extensions & benchmarks #4396
• GC improvements 3: turn datastore columns into ring-buffers #4397
• GC improvements 4: index EntityPathHashes in metadata registry #4398
• GC improvements 5: Store{Diff,Event} optimizations #4399
• GC improvements 6: introduce batched GC #4400
• GC improvements 7: introduce time_budget GC setting #4401

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[Wumpf]

lgtm, but you didn't lie, this one was a bit tougher!