Define GET /reference/{id}/bases in favor of GAReference.sequence.

This approach is intended to allow for more efficient access to the bases needed by the client. Returning an upwards of 300MB (uncompressed) string as a single JSON value would likely have performance issues; this is solved here via pagination if the endpoint chooses to implement the method with multiple pages.
This should be `{ long , null }` else it will throw a runtime exception IIRC (default value doesn't match default type).