WorkQueueThread: rework Cancel/Shutdown workflow

 - Cancel doesn't shut down anymore.
   Allowing it to be used multiple times thoughout the life of
   the WorkQueue
 - Remove Clear, so we only have Cancel semantics
 - Add IsCancelling so work items can abort early if cancelling
 - Replace m_cancelled and m_thread.joinable() guars with m_shutdown.
 - Rename Flush to WaitForCompletion (As it's ambiguous if a function
   called flush should be blocking or not)
 - Add documentation

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