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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@413088 f38db490-d61c-443f-a65b-d21fe96a405b