rtp_engine: Dynamic payload change in rtp mapping not supported

Asterisk didn't support the dynamic payload change in rtp mapping in the 200
OK response.

Scenario:
Asterisk sends the INVITE proposing alaw and telephone-event, it proposes
rtpmap:101 for telephone-event.  Peer responds with 2xx, it answers with
alaw and telephone-event also, but it proposes a different rtpmap number
(rtpmap:103) for telephone-event.

Expected Behaviour:
Asterisk should honour the rtpmapping in the response and send DTMF packets
using 103 as payload type for DTMF.

Actual Behaviour: Asterisk sends DTMF packets using payload type 101.

With this patch asterisk now supports changes that can occur in the rtp mapping
in the response.

(closes issue ASTERISK-23279)
Reported by: NITESH BANSAL
Review: https://reviewboard.asterisk.org/r/3225/
Patches:
     dynamic_payload_change.patch uploaded by nbansal (license 6418)
........

Merged revisions 408729 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 408730 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408731 65c4cc65-6c06-0410-ace0-fbb531ad65f3