refactor(driver-compat) remove o.b.BSONCallback#gotDBRef
feat(driver-compat) cleanup lazy stuff

Changes:
- retain fields order in lazy objects
- fix lazy objects to return j.u.Date instead of long
- fix equals() on LazyBSONObject
- hide several methods (package->private)
- add missing reset() to LazyBSONCallback
test(driver-compat) replace LazyBSONObject test with specs