PYTHON-803 - Don't use Collection.find() for commands.

This change introduces a _command helper in the helpers
module, used by Database.command and various internal and
external command helpers instead of iterating a Cursor
instance.

This change also introduces a small incompatibility with
earier versions. The as_class and fields options have been
removed from Database.command. They were never documented
and only used by some internal helpers. This also removes
the documented uuid_subtype option. A forthcoming API
refactor will replace that functionality.