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_super_block.go

@@ -2,7 +2,6 @@ package storage
 
 import (
 	"code.google.com/p/weed-fs/go/glog"
-	"code.google.com/p/weed-fs/go/util"
 	"fmt"
 	"os"
 )
@@ -15,12 +14,13 @@ const (
 * Super block currently has 8 bytes allocated for each volume.
 * Byte 0: version, 1 or 2
 * Byte 1: Replica Placement strategy, 000, 001, 002, 010, etc
-* Byte 2 and byte 3: Time to live in minutes
+* Byte 2 and byte 3: Time to live. See TTL for definition
+* Rest bytes: Reserved
  */
 type SuperBlock struct {
 	version          Version
 	ReplicaPlacement *ReplicaPlacement
-	Ttl              uint16
+	Ttl              *TTL
 }
 
 func (s *SuperBlock) Version() Version {
@@ -30,7 +30,7 @@ func (s *SuperBlock) Bytes() []byte {
 	header := make([]byte, SuperBlockSize)
 	header[0] = byte(s.version)
 	header[1] = s.ReplicaPlacement.Byte()
-	util.Uint16toBytes(header[2:4], s.Ttl)
+	s.Ttl.ToBytes(header[2:4])
 	return header
 }
 
@@ -70,6 +70,6 @@ func ParseSuperBlock(header []byte) (superBlock SuperBlock, err error) {
 	if superBlock.ReplicaPlacement, err = NewReplicaPlacementFromByte(header[1]); err != nil {
 		err = fmt.Errorf("cannot read replica type: %s", err.Error())
 	}
-	superBlock.Ttl = util.BytesToUint16(header[2:4])
+	superBlock.Ttl = LoadTTLFromBytes(header[2:4])
 	return
 }