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: http://svn.asterisk.org/svn/asterisk/trunk@417233 f38db490-d61c-443f-a65b-d21fe96a405b