bridges: Remove reliance on stasis caching

* The bridging core no longer uses the stasis cache for bridge
  snapshots.  The latest bridge snapshot is now stored on the
  ast_bridge structure itself.

* The following APIs are no longer available since the stasis cache
  is no longer used:
    ast_bridge_topic_cached()
    ast_bridge_topic_all_cached()

* A topic pool is now used for individual bridge topics.

* The ast_bridge_cache() function was removed since there's no
  longer a separate container of snapshots.

* A new function "ast_bridges()" was created to retrieve the
  container of all bridges.  Users formerly calling
  ast_bridge_cache() can use the new function to iterate over
  bridges and retrieve the latest snapshot directly from the
  bridge.

* The ast_bridge_snapshot_get_latest() function was renamed to
  ast_bridge_get_snapshot_by_uniqueid().

* A new function "ast_bridge_get_snapshot()" was created to retrieve
  the bridge snapshot directly from the bridge structure.

* The ast_bridge_topic_all() function now returns a normal topic
  not a cached one so you can't use stasis cache functions on it
  either.

* The ast_bridge_snapshot_type() stasis message now has the
  ast_bridge_snapshot_update structure as it's data.  It contains
  the last snapshot and the new one.

* cdr, cel, manager and ari have been updated to use the new
  arrangement.

Change-Id: I7049b80efa88676ce5c4666f818fa18ad1985369

tests/test_cel.c

@@ -351,7 +351,7 @@ static struct ast_str *__test_cel_generate_peer_str(struct ast_channel_snapshot
 static struct ast_str *test_cel_generate_peer_str_snapshot(struct ast_channel_snapshot *chan, struct ast_bridge *bridge)
 {
 	RAII_VAR(struct ast_bridge_snapshot *, snapshot,
-		ast_bridge_snapshot_get_latest(bridge->uniqueid),
+		ast_bridge_get_snapshot(bridge),
 		ao2_cleanup);
 
 	if (!snapshot) {