Allow passing a root folder to get the used space from in the quota wrapper

By getting the used space directly from `/files` for the Home storage we prevent having to make 2 calls to the filesystem case in the home cache (see [homecache.php](https://github.com/owncloud/core/blob/master/lib/private/files/cache/homecache.php#L42))

This saves one call to the cache for each time the quota wrapper needs to get the used space (write, copy and free_space)

cc @PVince81 @DeepDiver1975 @karlitschek