Add consumer-thread-create and consumer-thread-join events.

Add consumer-thread-create and consumer-thread-join events.

If an app listens to these, it can override the implementation of thread
creation and joining. Otherwise, if no listeners, it falls back to
pthread_create() and pthread_join() as usual. At this time, only the
base mlt_consumer uses this for  real_time=1 or -1 only.