Found through inspection. maxStalenessMS is used for selecting replica set members, but isn't sent to mongos.

https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#passing-read-preference-to-mongos

Fix that, and implement these tests:

https://github.com/mongodb/specifications/blob/master/source/max-staleness/max-staleness-tests.rst#direct-connection-to-mongos