Export BucketType from top-level namespace.
Add unit tests and fix class/instance variable problems.
Refactor client operations a little.

Made fetch_datatype return a real Datatype instance, while its lowdash
counterpart will only fetch the raw data.
Counters can be longs too
Fix up some more datatypes stuff.

- Add unit test to check the to_op() output.
- Fix the to_op() return values for flags and registers
- Prune adds from the Map.to_op() output if the field is updated.