add TTL support

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

go/proto/system_message.proto

@@ -10,6 +10,7 @@ message VolumeInformationMessage {
   optional bool read_only = 7;
   required uint32 replica_placement = 8;
   optional uint32 version = 9 [default=2];
+  optional uint32 ttl = 10;
 }
 
 message JoinMessage {