Fixed Confbridge file recording deadlock and appending.

A deadlock occurred after starting/stopping and then restarting a confbridge
recording.  Upon starting a recording a record thread is created that holds a
lock until just before exiting.  Stopping the recording does not stop/exit the
thread or release the lock.  The thread waits until recording begins again.
Starting a stopped recording signals the thread to continue and start recording
again.  However restarting the recording also created another record thread
resulting in a deadlock.  The fix was to make sure the record thread was only
created once.

Also it was noted that filenames for the recordings were being concatenated for
each start/stop.  This was fixed by creating a new file for each conference
session and appending the actual recorded data within the file (e.g. passing
the 'a' option to MixMonitor).

(issue AST-1088)
Reported by: John Bigelow
Review: http://reviewboard.digium.internal/r/374/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@381702 f38db490-d61c-443f-a65b-d21fe96a405b