fix liveness / performance bug in PooledThreadExecutor

[jmklix]

Issue #, if available: #2744
rebase of: #2745

Description of changes: Ensures that ThreadTask holds the PooledThreadExecutor's mutex between when it sees that the m_executor does not have any tasks, and when it waits on the condition variable.

Since AWS Semaphore does not take a mutex as parameter, replaces use of Semaphore with std::condition_variable here.

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