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.
If you had to read the PR then it wasn't clear enough from the docs. I've now explicitly called out pagination here to make the intended use obvious.