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/storage/volume_info.go

@@ -8,6 +8,7 @@ type VolumeInfo struct {
 	Id               VolumeId
 	Size             uint64
 	ReplicaPlacement *ReplicaPlacement
+	Ttl              *TTL
 	Collection       string
 	Version          Version
 	FileCount        int
@@ -32,5 +33,6 @@ func NewVolumeInfo(m *operation.VolumeInformationMessage) (vi VolumeInfo, err er
 		return vi, e
 	}
 	vi.ReplicaPlacement = rp
+	vi.Ttl = LoadTTLFromUint32(*m.Ttl)
 	return vi, nil
 }