common.BaseObject should be immutable

Currently MongoClient, Database, and Collection can be mutated through the BaseObject attributes read_preference, write_concern, uuid_subtype, etc. This is leading to problems in applications where different threads apply conflicting changes to the same object (reports of buggy applications like this are increasing).

PyMongo 3 will make instances of these attributes immutable. The existing attributes will become read only, uuid_subtype will be removed with its functionality folded into a CodecOptions class, tag_sets will be removed with its functionality folded into read preference, and a new API will be provided to create Database and Collection instances with explicit CodecOptions, ReadPreference, and WriteConcern.