Add snapshot cache that indexes by channel name

This adds a new channel snapshot cache in parallel to the existing
cache; the difference being that it indexes the channel snapshots by
channel name instead of channel uniqueid.

Review: https://reviewboard.asterisk.org/r/2576


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390249 65c4cc65-6c06-0410-ace0-fbb531ad65f3