DATACMNS-491 - Add support for configuring null handling hints in Sort.Order.

Null handling hints can now be configured on Order as being either:
NATIVE: we let the data store decide how to order nulls or NULLS_FIRST or
NULLS_LAST.
This is the foundation for custom null-handling in other data store support modules like SD JPA and SD MongoDB.

Original pull request: #79.