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: http://svn.asterisk.org/svn/asterisk/trunk@390249 f38db490-d61c-443f-a65b-d21fe96a405b
Remove remnant of snapshot blob JSON types

Remove usage of the once-mandatory snapshot blob type field, refactor
confbridge stasis messages accordingly, and remove
ast_bridge_blob_json_type().

Review: https://reviewboard.asterisk.org/r/2575/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@390250 f38db490-d61c-443f-a65b-d21fe96a405b