rtmp2: Lock self->lock before OBJECT_LOCK

rtmp2: Lock self->lock before OBJECT_LOCK

OBJECT_LOCK is used to protect property access only. self->lock is
used to access the RtmpConnection, mostly between the streaming thread
and the loop thread.

To avoid deadlocks involving these two locks, we obey a lock order:
If both self->lock and OBJECT_LOCK are needed, self->lock must be locked
first. Clarify this.