sync: add PoolStarvation benchmark
sync: add PoolStarvation benchmark

This benchmark simulates object starvation in order to force Ps to steal
objects from other Ps. Extracted from CL 314229.

Change-Id: Iee31df355ba04d80fbd91c4414e397a375e6d6d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/360256
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

CAFxX/go,golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go