Backend authentication fails with schemarouter

The schemarouter sometimes fails to properly clear the client session's default database resulting in errors from the backend if the database doesn't exist.

A good example is when a user connects to the schemarouter with a default database of db1. This database does not exist on all of the backend servers so to allow connections to all shards, the schemarouter clears the client protocols default database flags. Later the schemarouter sends a USE db1 query to the correct shard.