Audit MongoClient properties

Things that will be removed:

• host (replaced by address, which avoids a race condition)
• port (replaced by address, which avoids a race condition)
• document class (replaced by codec_options)
• the previously deprecated get_document_class method (replaced by codec_options)
• tz_aware (replaced by codec_options)

The documentation for "primary" and "secondaries" should also say what they return if not connected to a replica set. Similar to the docs for "arbiters".