DTMF start/end and hold/unhold events have state because a DTMF begin
event and hold event must be ended by something.

The following cases need to be handled when a channel is moved around in
the system.

* When a channel leaves a bridge it may owe a DTMF end event to the
bridge.

* When a channel leaves a bridge it may owe an UNHOLD event to the bridge.
(This case is explicitly ignored because things like transfers need
explicit control over this.)

* When a channel leaves the bridging system it may need to simulate a DTMF
end event to the channel.

* When a channel leaves the bridging system it may need to simulate an
UNHOLD event to the channel.

The patch also fixes the following:
* Fixes playing a file and restarting MOH using the latest MOH class used.

(closes issue ASTERISK-22043)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/2791/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397577 65c4cc65-6c06-0410-ace0-fbb531ad65f3