Race condition in Observer.start() and observer.schedule()

[matthew11]

There is a race condition between the two method. If the start() of the observer is called (but not finished, for example started by another thread) and in the mean time a schedule is called it can happen that the watcher never catches any event, basically silently failing the initialization.
The obvious solution could be to put the body of the start() behind the self._lock. After this I was not able to reproduce the race condition again.

Since the initialization done before the thread's actual start() called, as a workaround I can re-arrange my code in order to do similar initialization avoiding the race condition, but this bug should be an easy fix.

[BoboTiG]

Would you be able and motivated to provide a PR with the fix + a test?