Handle waiting_for_updates in callback

COUCHDB-2724
diff --git a/src/cassim_metadata_cache.erl b/src/cassim_metadata_cache.erl
index 920f748..ceb6c9e 100644
--- a/src/cassim_metadata_cache.erl
+++ b/src/cassim_metadata_cache.erl
@@ -162,6 +162,8 @@
     fabric:changes(DbName, fun changes_callback/2, Since, Args).
 
 
+changes_callback(waiting_for_updates, Acc) ->
+    {ok, Acc};
 changes_callback(start, Since) ->
     {ok, Since};
 changes_callback({stop, EndSeq}, _) ->