Add stateful PJSIP response API call, and use it for out-of-dialog responses.

Add stateful PJSIP response API call, and use it for out-of-dialog responses.

Asterisk had an issue where retransmissions of MESSAGE requests resulted in
Asterisk processing the retransmission as if it were a new MESSAGE request.

This patch fixes the issue by creating a transaction in PJSIP on the incoming
request. This way, if a retransmission arrives, the PJSIP transaction layer
will resend the response and Asterisk will not ever see the retransmission.

ASTERISK-24920 #close
Reported by Mark Michelson

Review: https://reviewboard.asterisk.org/r/4532/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@433619 65c4cc65-6c06-0410-ace0-fbb531ad65f3