Migrate PeerStatus events to stasis, add stasis endpoints, and add chan_pjsip device state.

(closes issue ASTERISK-21489)
(closes issue ASTERISK-21503)

Review: https://reviewboard.asterisk.org/r/2601/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392538 f38db490-d61c-443f-a65b-d21fe96a405b