Refactor operations to access the stasis cache instead of objects directly when retrieving information.

(closes issue ASTERISK-21883)

Review: https://reviewboard.asterisk.org/r/2645/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393831 65c4cc65-6c06-0410-ace0-fbb531ad65f3