Enable link-time optimizations

[olix0r]

This change enables LTO for release builds.

This increases compile-time overhead, so we also update rust-toolchain to 1.55.0 beta7 (due out 2021/9/9), which substantially reduces compile-time overhead.

Comparison: main / lto / mimalloc / lto+mimalloc

load test run as:

helm upgrade ort ./chart -n ort \
    --set load.threads=3 \
    --set load.flags.concurrencyLimit=$((3*3)) \
    --set load.flags.requestLimit=10000 \
    --set server.threads=2 \
    --set server.services=2 \
    --set linkerd.config.proxyCpuLimit=3 \
   ...

LTO reduces CPU and memory overhead without noticeably impacting latency. mimalloc helps server latency but impacts cpu & memory usage. mimalloc+lto uses least CPU+memory but exhibits poor tail latency on the server.

[olix0r]

release build timing:

1.54.0

    User time (seconds): 1526.52
    System time (seconds): 39.11
    Percent of CPU this job got: 615%
    Elapsed (wall clock) time (h:mm:ss or m:ss): 4:14.18
    Maximum resident set size (kbytes): 11790572

1.55.0-beta.7

    User time (seconds): 1126.32
    System time (seconds): 27.04
    Percent of CPU this job got: 568%
    Elapsed (wall clock) time (h:mm:ss or m:ss): 3:22.72
    Maximum resident set size (kbytes): 4532484

1.55.0-beta.7 +lto

    User time (seconds): 746.15
    System time (seconds): 20.96
    Percent of CPU this job got: 266%
    Elapsed (wall clock) time (h:mm:ss or m:ss): 4:47.70
    Maximum resident set size (kbytes): 3889984

[hawkw]

whoa, cool! i thought we already did this, but maybe i was misremembering.