Eliminate timeout in waiting on event queue

[sattlerc]

The use of a timeout in reading from the event queue in BaseObserver is a hack. This causes an unnecessary wake-up every second and causes the following program to take one second:

from watchdog.observers import Observer
observer = Observer()
observer.start()
observer.stop()
observer.join()

This pull request gets rid of the timeout by adding a null event to the event queue after a stop is requested.

Note that, contrary to documentation, this timeout value was also used for other things than reading from the event queue. It is also passed to emitters constructed in BaseObserver. I have thus left the timeout attribute in place and instead fixed its documentation.

[BoboTiG]

Thanks @sattlerc. Do you mind adding a line + your GitHub username in the changelog.rst file 🙏 ?