crimson/osd: let OmapIterator mutators return future<>

crimson/osd: let OmapIterator mutators return future<>

instead of returning future<int> where the int represents an errno which
is not checked by any callers, we should just return a future<>, we
could erroratorize it later on, but returning an int is not the right
way to handle this, and it is not consistent with how we handle other
errors elsewhere in crimson.

Signed-off-by: Kefu Chai <kchai@redhat.com>