res_pjsip: Add support for building against pjproject with SIP transaction group lock support.

SIP transaction group lock support has been backported into our pjproject. Since the code
now internally uses a group lock the code is now changed to unlock it if present. Note
that the act of finding the transaction is what actually returns it locked.

For further information about group locks check out the wiki page at:
http://trac.pjsip.org/repos/wiki/Group_Lock

(issue ASTERISK-22818)
Reported by: Matt Jordan
........

Merged revisions 402864 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402865 65c4cc65-6c06-0410-ace0-fbb531ad65f3