Harmonize the behavior of getMore when called on a cursor with a wrong namespace: This ticket changes mongos to have the same validateNamespace check as mongod has, returning ErrorCodes::Unauthorized in case the namespace validation fails.

Description of Linked Ticket

We have a check on mongod, validateNamespce, that checks whether the user is issuing a getMore against the same namespace than the initial cursor was established on. On mongod, doing this will hit this check and cause a failure. On mongos, we will just look up the cursor and ignore the the provided namespace, and re-write the request with the correct namespace. This isn't an issue security-wise, as we still have an auth check for the read privileges on the mongos, but it is a bit weird from a UX perspective to have different behaviors here.