This adds a `wait_for_completion` flag which allows the user to block
the Stop API until the task has actually moved to a stopped state,
instead of returning immediately. If the flag is set, a `timeout` parameter
can be specified to determine how long (at max) to block the API
call. If unspecified, the timeout is 30s.
If the timeout is exceeded before the job moves to STOPPED, a
timeout exception is thrown. Note: this is just signifying that the API
call itself timed out. The job will remain in STOPPING and evenutally
flip over to STOPPED in the background.
If the user asks the API to block, we move over the the generic
threadpool so that we don't hold up a networking thread.
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