More efficient bucket resolution

Addressed by #940 .
Nice and reasonable shortcut for no siblings.

What do you think to make it a bit more general ? :

```
resolve_buckets([HeadUserRec|RestUserRecs], [], KeepDeletedBuckets) ->
    resolve_buckets(RestUserRecs, UserRec?RCS_USER.buckets, KeepDeletedBuckets).
```