statsvfs: support filesystem statistics #36
Description
Implement a way to keep filesystem statistics (blocks used, bytes used, etc.)
Keep information in cache and save it not too often to not kill overwrite on zdb level.
Implement a kind of fsck to re-sync statistics, to be called after a crash or something like that.
Current implementation rely on zdb namespace usage, which is quiet accurate but maybe not enough.
This is in addition with #21