Fix crash from bridge channel hangup race condition in ConfBridge

Fix crash from bridge channel hangup race condition in ConfBridge

This patch addresses two issues in ConfBridge and the channel bridge layer:
1. It fixes a race condition wherein the bridge channel could be hung up
2. It removes the deadlock avoidance from the bridging layer and makes the
   bridge_pvt an ao2 ref counted object

Patch by David Vossel (mjordan was merely the commit monkey)

(issue ASTERISK-18988)
(closes issue ASTERISK-18885)
Reported by: Dmitry Melekhov
Tested by: Matt Jordan
Patches: chan_bridge_cleanup_v.diff uploaded by David Vossel (license 5628)

(closes issue ASTERISK-19100)
Reported by: Matt Jordan
Tested by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/1654/
........

Merged revisions 350550 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@350551 65c4cc65-6c06-0410-ace0-fbb531ad65f3