Issue 2744: fix liveness / performance bug in PooledThreadExecutor

[hunjmes]

Issue #, if available: 2744

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.

Check all that applies:

• [ x ] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [ x ] Checked if this PR is a breaking (APIs have been changed) change.
• [ x ] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [ x ] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [ x ] Linux
• [ ] Windows
• [ ] Android
• [ ] MacOS
• [ ] IOS
• [ ] Other Platforms

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