app_confbridge: Repeatedly starting and stopping recording ref leaks the recording channel.

Starting and stopping conference recording more than once causes the
recording channels to be leaked.  For v13 the channels also show up in the
CLI "core show channels" output.

* Reworked and simplified the recording channel code to use
ast_bridge_impart() instead of managing the recording thread in the
ConfBridge code.  The recording channel's ref handling easily falls into
place and other off nominal code paths get handled better as a result.

ASTERISK-24719 #close
Reported by: John Bigelow

Review: https://reviewboard.asterisk.org/r/4368/
Review: https://reviewboard.asterisk.org/r/4369/
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Merged revisions 431135 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 431160 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431161 65c4cc65-6c06-0410-ace0-fbb531ad65f3