[C++] - Fix shutdown race condition

[jbcm1234]

A race condition exists in the channel shutdown code between the thread calling WorkStealingThreadPoolImpl::Quiesce() and a worker thread in WorkStealingThreadPool::ThreadState::Step().

Referring to these threads as thread A (Quiesce thread) and thread B (Step thread),
it is possible that the following sequence occurs:

• Thread A calls SetShutdown(true)
• Thread B does not see the update to shutdown_ before waiting on the work signal.
• Before thread B waits on the work signal with WaitWithTimeout thread A broadcasts the signal.
• Thread B now enters WaitWithTimeout but has to wait for the timeout to expire as the signal / condition variable was signalled before it started waiting on it.

This timeout can be in the order of seconds and will stall the thread calling Quiesce until the worker thread (thread B in the above scenario) exits.

[linux-foundation-easycla]

• ❌ - login: @jbcm1234 / name: Jacques Fourie . The commit (9f670c9) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[markdroth]

We can't look at this until you sign the CLA.

[markdroth]

The CLA still hasn't been signed, so we can't look at this. Feel free to open a new PR once you can sign the CLA.