[internal] Make GIL usage explicit for session_poll_workunits

[Eric-Arellano]

Prework for #13526. We were reacquiring the GIL directly inside py.allow_threads(||). Rust-cpython allows that, but PyO3 does not. (PyO3 has the benefit of instilling much stronger confidence in reasoning when the GIL is held or not.)

We can easily work around this by having WorkunitStore.with_latest_workunits return Vec<Workunit> instead of &[Workunit]. No new clones are happening! We already were making this Vec. Only now we move the value out, rather than referencing it via a closure passed in as a callback.

That change allows for operating on the Vec<Workunit> safely outside of the py.allow_threads(||) to convert it into the FFI boundary.

Benchmark

These benchmarks use the Toolchain BuildSense plugin + anonymous telemetrics streaming workunit handlers.

No pantsd

Before:

❯ hyperfine -w 1 -r 10 './pants -ldebug --no-pantsd list ::'
  Time (mean ± σ):      4.611 s ±  0.206 s    [User: 2.940 s, System: 0.474 s]
  Range (min … max):    4.419 s …  5.153 s    10 runs

After:

❯ hyperfine -w 1 -r 10 './pants -ldebug --no-pantsd list ::'
  Time (mean ± σ):      4.655 s ±  0.113 s    [User: 2.975 s, System: 0.509 s]
  Range (min … max):    4.430 s …  4.827 s    10 runs

Pantsd

Before:

❯ hyperfine -w 1 -r 10 './pants -ldebug --concurrent --pantsd list ::'
  Time (mean ± σ):      2.641 s ±  0.021 s    [User: 2.026 s, System: 0.323 s]
  Range (min … max):    2.602 s …  2.670 s    10 runs

After:

❯ hyperfine -w 1 -r 10 './pants -ldebug --concurrent --pantsd list ::'
  Time (mean ± σ):      2.651 s ±  0.023 s    [User: 2.036 s, System: 0.334 s]
  Range (min … max):    2.611 s …  2.693 s    10 runs

[ci skip-build-wheels]