Deprecate loadDb() in favor of __get magic method access

This change will reduced the risk of errors. Forgeting to
call loadDb before accessing it, results in a php error, which can be
avoided by just accessing the requsted database.

This also works well with mongodb's nature, where anything that is
requested is created.
Remove old code that is no longer required

- Remove addDb()
- Remove un-used parameters from __construct() and configure()
- Remove slaveOk (no longer used in Mongo or MongoClient)
- Remove configure() database loading section
Remove identical code already defined in base class