Confbridge channels staying active when all participants leave.

If you started/stopped recording of a conference multiple times channels
would remain active even when all participants left the conference.  This
was due to the fact that a reference to the confbridge was being added
every time a start record command was issued, but when the recording was
stopped there was no matching de-reference thus keeping the conference alive.
Made sure only a single reference is added for the record thread no matter how
many times recording is started/stopped.  A de-reference is issued upon thread
ending.

Note, this issue is being fixed under AST-1088 since it relates to it and
should have been corrected along with those modifications.

(issue AST-1088)
Reported by: John Bigelow


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@381737 65c4cc65-6c06-0410-ace0-fbb531ad65f3