Abstract PJSIP-specific elements from the pubsub API.

This helps to pave the way for RLS work that is to come.
Since this is a self-contained change and subscription
tests still pass, this work is being committed directly
to trunk instead of a working branch.

ASTERISK-23865 #close
Review: https://reviewboard.asterisk.org/r/3628



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417233 65c4cc65-6c06-0410-ace0-fbb531ad65f3