rgw: data changes log, don't always send new requests

We may piggy back on older entries that hasn't expired yet.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
lru_map: infrastructure for a bounded map

Useful for cache-like maps, where we want to control
the max number of entries in the map.

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