Avoid deep copy of numpy buffers on unspill

[crusaderky]

• Closes Deep copy causes memory flare on unspill #7407
• Must be merged together with Avoid deep copy of numpy buffers on unspill zict#74

Benchmark

import gc
import numpy
import zict
from distributed.spill import SpillBuffer

print(zict.__version__)
a = numpy.random.random(512 * 2**20 // 8)
buf = SpillBuffer("/tmp", target=1)
buf["xxx"] = a
buf.slow.cache.clear()  # Clear weakref cache
buf["xxx"]
buf.get_metrics()

Output:

2.2.0
{...
 'unpickle_time_total': 0.07338492600138125}

2.3.0
{...
 'unpickle_time_total': 0.00016480999875057023}

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       22 files  ±0         22 suites  ±0   9h 55m 14s ⏱️ - 23m 21s
  3 278 tests ±0    3 188 ✔️ ±0       86 💤 ±0  4 ❌ ±0 
35 989 runs  ±0  34 466 ✔️  - 1  1 518 💤 ±0  5 ❌ +1 

For more details on these failures, see this check.

Results for commit 987d6aa. ± Comparison against base commit 129f62a.

♻️ This comment has been updated with latest results.

[crusaderky]

Peer review on dask/zict#74

[crusaderky]

CI failures are due to conda env caching - should disappear tomorrow