RTP: reset DTMF last seqno/timestamp on RTP renegotiation

The remote side may start a new stream when renegotiating RTP.
Need to reset the DTMF last sequence number and the timestamp
of the last END packet on RTP renegotiation.

If the new time stamp is lower then the timestamp of the last DTMF END packet
the asterisk drops all DTMF frames as out of order.

This bug was caught using Cisco ip-phone SPA5XX and codec g722.
On SIP session update the SPA50X resets stream and a new timestamp is twice
smaller then the previous.

ASTERISK-28162 #close

Change-Id: Ic72b4497e74d801b27a635559c1cf29c16c95254