[Snapshot] Implement new quota including space exclusively used in snapshots

(Excerpt from the design doc)

g) Quota considerations:

1. Currently, quota usage is immediately decreased when a key is successfully confirmed deleted in OM, rather than when its key blocks are reclaimed. As a result, if we do not change the logic in validateAndUpdateCache in OMKeyDeleteRequest (and the FSO equivalent) to make it aware of snapshots, effectively those key space exclusively used in snapshots won’t count against the quota, because their quota usage is “released” the minute the keys are deleted from the active FS.

2. On one hand, it sounds reasonable to leave the current quota handling logic in the active file system as-is, because in a case where a regular user Bob has exhausted its bucket quota and at this point an admin Alice takes a snapshot on the bucket, Bob would have no way to release his bucket quota usage if snapshot usage also counts against the current Ozone quota implementation. Only admins are able to delete and create snapshots.

3. If needed, we could introduce another kind of quota which includes snapshot usage in the future, similar to what ZFS has. ZFS has refquota which doesn't count snapshot usage (like what we currently have in Ozone), and quota which counts snapshot usage (and others) as well.