chan_pjsip: Implement core ability to get Call-ID of a channel.

This changes implement the "get_pvt_uniqueid" which is used to return the
technology specific unique identifier. In the case of SIP this is the Call-ID
of the dialog.

Review: https://reviewboard.asterisk.org/r/3480/
........

Merged revisions 413088 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@413089 65c4cc65-6c06-0410-ace0-fbb531ad65f3