reset timestamp counter when we get new sdp etc because sonus likes to say ptime 20 and send 30ms timestamps in the 183 then once they say 200 ok with the same sdp they decide to actually send 20 for real this time FSRTP-8

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@15597 d0543943-73ff-0310-b7d9-9358b9ac24b2