Implement mark_clean for ddf and remove mark_dirty and mark_sync

Implement mark_clean for ddf and remove mark_dirty and mark_sync

mark_dirty is just a special case of mark_clean - with sync_pos == 0.
mark_sync is not required.  We don't modify the metadata when sync
finishes.  Only when the array becomes non-writeable at which point we
use mark_clean to record how far the resync progressed.