bridge_channel.c: Fix FRACK when mapping frames to the bridge.

* Add protection checks when mapping streams to the bridge.  The channel
and bridge may be in the process of updating the stream mapping when a
media frame comes in so we may not be able to map the frame at the time.

* We need to map the streams to the bridge's stream numbers right before
they are written into the bridge.  That way we don't have to keep
locking/unlocking the bridge and we won't have any synchronization
problems before the frames actually go into the bridge.

* Protect the deferred queue with the bridge_channel lock.

ASTERISK-27212

Change-Id: Id6860dd61b594b90c8395f6e2c0150219094c21a