objectstore: make list by hash *next > instead of >=

objectstore: make list by hash *next > instead of >=

This means we should set it to a hash boundary or the last item of our
result set (not the next item we didn't include).

It means that during backfill we can set our last_backfill to the last
object we did recover and be sure that any new files locally will be
included in the next result set, and we can bound that result set by that
last object recovered and not include it in the resulting range.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>