[MONDRIAN-1998] Followup to the previous commits of this case. It was later found that the problem isn't about whether we delete the index from the registry or not at the end of the life of a schema, but rather a cache miss. It is perfectly fine to base the key of the registry off the schema's checksum, as it is also preferable. When we bring a schema offline, we already clean all the finished segments and also use the deletion flag to do a graceful recovery for the segments that were still loading at the time. Deleting the index prevents this graceful shutdown and causes the user thread to hang, as we sometimes delete the index from the registry before all of the queries are terminated. By the time they finish, the star was re-created and doesn't have the same object ident anymore, so we can't notify the user threads that the query has finished.