Add more support for native bridging.

* Added a start technology callback that technologies can use to start
bridging operations.  It is expected that native bridges will find this
useful.

* Factored out bridge_channel_complete_join().


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390991 65c4cc65-6c06-0410-ace0-fbb531ad65f3