res_pjsip_messaging: Serialize outbound SIP MESSAGEs

Outbound SIP MESSAGEs had the potential to be sent out
of order from how they were specified in a set of
dialplan steps.

This change creates a serializer for sending outbound
MESSAGE requests on. This ensures that the MESSAGEs are
sent by Asterisk in the same order that they were sent
from the dialplan.

ASTERISK-24937 #close
Reported by Mark Michelson

Review: https://reviewboard.asterisk.org/r/4579
........

Merged revisions 433968 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433969 65c4cc65-6c06-0410-ace0-fbb531ad65f3