Originally there are only url(ip + port), and publicUrl. Because ip was
used to listen for http service, it has less flexibility and volume
server has to be accessed via publicUrl.
Recently we added ip.bind, for binding http service.
With this change, url can be used to connect to volume servers. And
publicUrl becomes a free style piece of url information, it does not
even need to be unique.
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
