Cleans the MOB files when they're expired and their min versions are 0. If the latest timestamp
of Cells in a MOB file is older than the TTL in the column family, it's regarded as expired.
This cleaner deletes them. At a time T0, the cells in a mob file M0 are expired. If a user
starts a scan before T0, those mob cells are visible, this scan still runs after T0. At that
time T1, this mob file M0 is expired, meanwhile a cleaner starts, the M0 is archived and can be
read in the archive directory.