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: http://svn.asterisk.org/svn/asterisk/trunk@390991 f38db490-d61c-443f-a65b-d21fe96a405b