Schemarouter should prefer targets which have databases in them for session commands

A backend can end up being chosen as the replier of a session command even before authentication to it has completed. For readwritesplit this is not a problem as the state of the server implies which server to choose but for the schemarouter there is nothing that tells which server to prefer.

An improvement to the situation can be achieved by looking into the shard map and seeing if any of the backends is known to have been successfully mapped. This should avoid most problems that could occur when a user exists on a subset of the servers.