rgw: sync bucket stats thread

a new thread that periodically sync stats of recently modified buckets.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: round bytes to the next 4k for quota use

Be more consistent as we round it like this in the objclass, so we need
the accounting to match.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>