GObject may crash if two threads do concurrent g_object_notify() on the same
object. This may happen if fakesink receives an out-of-band event such as
FLUSH_START while processing a buffer or serialised event in the streaming
thread. Since this may happen with the default settings during a common
operation like a seek, and there seems to be little chance of a timely fix
in GObject (see #166020), we should hack around this issue by protecting all
of fakesink's direct g_object_notify() calls with a lock.

Also add unit test for the above.

Fixes #554460.