Summary:
This draining mechanism should not be run during `JoinThreads()` because it can detach threads that will be joined. Joining detached threads would throw an exception.
With this PR, we skip draining when `JoinThreads()` has already decided what threads to `join()`, so the threads will exit naturally once the work queue empties.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8635
Test Plan: verified it unblocked using `WaitForJobsAndJoinAllThreads()` in https://github.com/facebook/rocksdb/issues/8611.
Reviewed By: riversand963
Differential Revision: D30174587
Pulled By: ajkr
fbshipit-source-id: 144966398a607987e0763c7152a0f653fdbf3c8b