- 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
6594532f10