cluster: implement stopcursor
farm: implement stopcursor
roshi-server: implement stopcursor
SelectCursor becomes SelectRange

`cursor` parameter becomes `start`, and `stopcursor` becomes `stop`.
Order is strictly newest-to-oldest.

Also, cursors for each element are given in every response, regardless
if it was a SelectOffset- or SelectRange-style query.
Merge pull request #25 from soundcloud/stopcursor

stopcursor