fallbackswitch: Move active_sinkpad out of State into its own Mutex
As described in issue #200 (closed), we hold the srcpad's stream lock in some
situations where we notify the active-pad property.
If there's a handler installed it will most likely attempt to read the
property, which had to take the state lock. Another thread could
already be holding this lock and attempting to obtain the srcpad's
stream lock. This resulted in a deadlock.
To avoid this, move the active_sinkpad field into its own Mutex, which
we never hold for long.
Fixes: #200 (closed)