res_rtp_asterisk: Address jittery DTMF events in RTP streams

res_rtp_asterisk: Address jittery DTMF events in RTP streams

(closes issue ASTERISK-21170)
Reported by: NITESH BANSAL
Patches:
    dtmf-timestamp.patch uploaded by NITESH BANSAL (license 6418)
Review: https://reviewboard.asterisk.org/r/2938/
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Merged revisions 401619 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 401620 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@401621 65c4cc65-6c06-0410-ace0-fbb531ad65f3