This supposedly should reduce memory consumption. However, for tests
with millions of, this shows consuming more memories. Need to see
whether this will work out. If not, later boltdb will be tested.
The volume TTL and file TTL are not necessarily the same. as long as
file TTL is smaller than volume TTL, it'll be fine.
volume TTL is used when assigning file id, e.g.
http://.../dir/assign?ttl=3h
file TTL is used when uploading