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