refactor(driver-compat) remove com.mongodb.DBRefBase

Changes:
- removed c.m.DBRefBase class;
- o.m.DBRef is now superclass of a c.m.DBRef;
- removed c.m.codecs.DocumentCodec as unnecessary;
- ported c.m.DBRefTest as c.m.DBRefOldTest;
- fixed number of issues came up from DBRefOldTest.
feat(driver-compat) cleanup c.m.ReadPreference

Changes:
- implemented getTagSets(), toDBObject() and toString() in c.m.TaggableReadPreference
- implemented toString() in c.m.ReadPreference
- ported c.m.ReadPreferenceTest to Spock
- added 2.x tests