Rename _cassim database to _metadata

COUCHDB-2620
diff --git a/src/cassim_metadata_cache.erl b/src/cassim_metadata_cache.erl
index f971243..85113e2 100644
--- a/src/cassim_metadata_cache.erl
+++ b/src/cassim_metadata_cache.erl
@@ -56,7 +56,7 @@
 
 
 metadata_db() ->
-    config:get("couchdb", "metadata_db", "_cassim").
+    config:get("couchdb", "metadata_db", "_metadata").
 
 
 metadata_db_exists() ->