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/needle_read_write.go

@@ -14,7 +14,9 @@ const (
 	FlagHasName             = 0x02
 	FlagHasMime             = 0x04
 	FlagHasLastModifiedDate = 0x08
+	FlagHasTtl              = 0x10
 	LastModifiedBytesLength = 5
+	TtlBytesLength          = 2
 )
 
 func (n *Needle) DiskSize() int64 {
@@ -70,6 +72,9 @@ func (n *Needle) Append(w io.Writer, version Version) (size uint32, err error) {
 			if n.HasLastModifiedDate() {
 				n.Size = n.Size + LastModifiedBytesLength
 			}
+			if n.HasTtl() {
+				n.Size = n.Size + TtlBytesLength
+			}
 		}
 		size = n.DataSize
 		util.Uint32toBytes(header[12:16], n.Size)
@@ -112,6 +117,12 @@ func (n *Needle) Append(w io.Writer, version Version) (size uint32, err error) {
 					return
 				}
 			}
+			if n.HasTtl() {
+				n.Ttl.ToBytes(header[0:TtlBytesLength])
+				if _, err = w.Write(header[0:TtlBytesLength]); err != nil {
+					return
+				}
+			}
 		}
 		padding := NeedlePaddingSize - ((NeedleHeaderSize + n.Size + NeedleChecksumSize) % NeedlePaddingSize)
 		util.Uint32toBytes(header[0:NeedleChecksumSize], n.Checksum.Value())
@@ -194,6 +205,10 @@ func (n *Needle) readNeedleDataVersion2(bytes []byte) {
 		n.LastModified = util.BytesToUint64(bytes[index : index+LastModifiedBytesLength])
 		index = index + LastModifiedBytesLength
 	}
+	if index < lenBytes && n.HasTtl() {
+		n.Ttl = LoadTTLFromBytes(bytes[index : index+TtlBytesLength])
+		index = index + TtlBytesLength
+	}
 }
 
 func ReadNeedleHeader(r *os.File, version Version, offset int64) (n *Needle, bodyLength uint32, err error) {
@@ -263,3 +278,9 @@ func (n *Needle) HasLastModifiedDate() bool {
 func (n *Needle) SetHasLastModifiedDate() {
 	n.Flags = n.Flags | FlagHasLastModifiedDate
 }
+func (n *Needle) HasTtl() bool {
+	return n.Flags&FlagHasTtl > 0
+}
+func (n *Needle) SetHasTtl() {
+	n.Flags = n.Flags | FlagHasTtl
+}