fix potential deadlock on shutdown

[sbiscigl]

Description of changes:

A customer was seeing a issue where ExclusiveOwnershipResourceManager would see a deadlock on shutdown. specifically in ShutdownAndWait which currently looks like

Aws::Vector<RESOURCE_TYPE> ShutdownAndWait(size_t resourceCount) {
  std::unique_lock<std::mutex> locker(m_queueLock);
  m_shutdown = true;
  ... drain all resources
}

and issue could arise if in Acquire

RESOURCE_TYPE Acquire() {
  std::unique_lock<std::mutex> locker(m_queueLock);
  while(!m_shutdown.load() && m_resources.size() == 0) {
    m_semaphore.wait(locker, [&](){ return m_shutdown.load() || m_resources.size() > 0; });
  }
  ...
}

where m_resources never dips to zero making it so that the queue lock is never released which would never allow for ShutdownAndWait to run. by swapping ShutdownAndWait to be

Aws::Vector<RESOURCE_TYPE> ShutdownAndWait(size_t resourceCount) {
  m_shutdown = true;
  std::unique_lock<std::mutex> locker(m_queueLock);
  ... drain all resources
}

we actually short circuit that while loops -- as intended. Also m_shutdown is atomic and can live outside of the lock.

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

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.