PYTHON-672 - Make read preference a class

This commit introduces the following changes:

- Each read preference is now a class.
- Read preferences other than Primary accept a tag_sets parameter.
- The tag_sets attribute of MongoClient, Database, and Collection has
  been removed. Use the tag_sets parameter of the new classes instead.
- The read_preferences.ReadPreference constant still exists and should
continue to work as expected for most users.