Update command routing for Server Selection Spec

Update PyMongo's database.command() method and various command helpers like aggregate(), group(), find_and_modify(), etc. to follow the new Server Selection Spec:

https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#use-of-read-preferences-with-commands

Database.command() already follows the spec in PyMongo 3: ignores the Database instance's read preference. Uses read preference mode PRIMARY by default, obeys whatever read preference the user explicitly configures.

Check over the command helpers like Collection.aggregate or Database.collection_names to ensure they follow the spec as well. Commit f6a260c6 brought us close to compliance.