This patch does the following:
 * It pulls out bridge_channel and puts it into its own translation unit
 * It adds public and protected headers for bridging_channel. Protected
   functions are appropriate only for the Bridging API and sub-classes of a
   bridge.

(issue ASTERISK-22130)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@395253 65c4cc65-6c06-0410-ace0-fbb531ad65f3