bridge_multiplexed: Keep the multiplexed thread until no more bridges use it.

bridge_multiplexed: Keep the multiplexed thread until no more bridges use it.

* Fixed the potential of losing the multiplexed bridge thread when the
last channel leaves and another joins while the multiplexed thread is
being shut down.

* Refactored and improved the management of the serviced channels array.

* Changed the channels count to a bridges count so it only needs to be
incremented rather than changed by two.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380666 65c4cc65-6c06-0410-ace0-fbb531ad65f3