chan_sip: complete upgrade to ao2

This change upgrades sip_registry and sip_subscription_mwi to astobj2.

ASTERISK-24067 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3759/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419438 65c4cc65-6c06-0410-ace0-fbb531ad65f3