ARI: Implement channel hold/unhold.

This puts the channel on hold (rather than queueing a frame from the channel).

(closes issue ASTERISK-21619)

Review: https://reviewboard.asterisk.org/r/2647/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393332 65c4cc65-6c06-0410-ace0-fbb531ad65f3