Performance degradation with context pool after v0.3.0

[adriantam]

Our project are using context pool (with WithContext(), WithCancelOnError(), WithFirstError() and WithMaxGoroutines()). After upgrading from v0.3.0 to [5f936ab](https://github.com/sourcegraph/conc/commit/5f936abd7ae87036af1f75c95fb9d0daaf00116b), we have observed performance degradation (significant increase in CPU). The usually pattern involves pool.New(), pool.Go() and pool.Wait() with high concurrency.

[srabraham]

@adriantam I will happily try to debug this. Could you please provide a sample piece of code that exhibits this behavior? This would be great in the form of a Benchmark test. https://pkg.go.dev/testing#hdr-Benchmarks