Parallelize WaitForJobChanges

As for QueryJobs we rely on file updates rather than condition
notification to acquire job changes. In order to do that we use the
pyinotify module to watch files. This might make the client a bit slower
(pending planned improvements, such as subscription-based
WaitForJobChanges) but detaches it from the job execution.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>