chan_pjsip: Update media translation paths when new SDP negotiated.

On a SIP reinvite that changes media strams, the PJSIP channel driver was
flooding the log with "Asked to transmit frame type %s, while native
formats is %s" warnings.

* Fixes PJSIP not setting up translation paths when the formats change on
a reinvite.  AFS-63 was effectively reintroduced because of the media
formats work.  res_pjsip_sdp_rtp.c:set_caps()

* Improved the unexpected frame format WARNING message to include more
information.

* Added protective locking while altering formats on a channel.  Reworked
set_format() to simplify and protect the formats under manipulation.

* Restored some code that got lost in the media_formats work.
(channel.c:set_format() and res_pjsip_sdp_rtp.c:set_caps())

AFS-137 #close
Reported by: Mark Michelson

Review: https://reviewboard.asterisk.org/r/3906/
........

Merged revisions 421645 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421646 65c4cc65-6c06-0410-ace0-fbb531ad65f3