Also refactor various internals of the monitor code:
- Don't allow starting twice but just return directly when starting a
second time.
- Don't end up in an inconsistent state if call start() a second time
while the monitor is starting up.
- Remove complicated cookie code: it was not possible to add/remove
filters while the monitor was started anyway so this was only useful
in the very small time-window while starting the monitor or while
getting the devices. Instead disallow adding/removing filters while
the monitor is starting, and when getting devices work on a snapshot
of providers/filters.
Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/667
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1189>
