rgw: rename RGWBucketStats to RGWStorageStats

make it more generic, will be reused for user's stats.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: refactor quota cache

bucket quota cache is going to be reused, add a new abstract layer.

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