In the past, the explain commands have been implicitly creating a database when the database didn't exist because they were instantiating the CollectionRoutingInfoTargeter on the mongos (which implicitly creates the db if it doesn't exist).

This made the explain operations run always against an existing database.

When we started using the CollectionRouter to coordinate the collection routing under SERVER-103705 and SERVER-103706, it was necessary to explicitly create the database as one of the first steps of the explain operation on the mongos.

The goal of this task is to be able to respond an explain command without creating the database if it doesn't exist.