Reset RTP timestamp; sequence number on SSRC change

In r370252 for ASTERISK-18404, Asterisk's handling of RTP was modified to
better account for out of order RTP packets. This was accomplished by using the
RTP timestamp and sequence number to check for out of order packets. However,
when a SSRC change occurs, the timestamp and sequence number will no longer
have any relation to the previously received packets. The variables tracking
the timestamp and sequence number therefore have to be reset.

(closes issue ASTERISK-20906)
Reported by: Eelco Brolman
patches:
  dtmf_on_hold.patch uploaded by Eelco Brolman (license #6442)
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Merged revisions 378967 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 378984 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@378985 f38db490-d61c-443f-a65b-d21fe96a405b