When processing RFC 2833 DTMF, accomodate increasing timestamps in End events

When processing RFC 2833 DTMF, accomodate increasing timestamps in End events

While endpoints should not be changing the source timestamp between DTMF event
packets, the fact is there exists those endpoints that do exactly that.  To
work around this, we absorb timestamps within the expected re-transmit period.
Note that this period only affects End of Event packets, so it should not
prevent the detection of new DTMF digits that happen to arrive right on top
of each other.

(closes issue ASTERISK-20424)
Reported by: Vladimir Mikhelson
Tested by: mjordan, Vladimir Mikhelson

Review: https://reviewboard.asterisk.org/r/2124
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Merged revisions 373236 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@373237 65c4cc65-6c06-0410-ace0-fbb531ad65f3