StopMixMonitor race condition (not giving up file immediately)

StopMixMonitor race condition (not giving up file immediately)

StopMixMonitor only indicates to the MixMonitor thread to stop
writing to the file.  It does not guarantee that the recording's
file handle is available to the dialplan immediately after execution.
This results in a race condition.  To resolve this, the filestream
pointer is placed in a datastore on the channel. When StopMixMonitor
is called, the datastore is retrieved from the channel and the
filestream is closed immediately before returning to the dialplan.
Documentation indicating the use of StopMixMonitor to free files
has been updated as well.

(closes issue #15259)
Reported by: travisghansen
Tested by: dvossel

Review: https://reviewboard.asterisk.org/r/283/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@201423 65c4cc65-6c06-0410-ace0-fbb531ad65f3