Dropping a scheduled runnable cancels its task and makes the next poll
of any awaiter panic with "Task polled after completion." The only paths
where we drop these runnables seem to be during shutdown or extreme
resource exhaustion, so, let's leak the runnables instead of crashing.

On Windows, we also moved to calling the Win32 thread pool API directly,
because 1) WinRT thread pool API is just a wrapper that adds overhead we
don't need, and 2) the closure we pass to the `WorkItemHandler` object
takes ownership of the runnable object, so if the WinRT thread pool
releases the delegate, it can free the runnable without our control.

Release Notes:

- N/A