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_vacuum.go

@@ -4,7 +4,7 @@ import (
 	"code.google.com/p/weed-fs/go/glog"
 	"fmt"
 	"os"
-	_ "time"
+	"time"
 )
 
 func (v *Volume) garbageLevel() float64 {
@@ -13,9 +13,10 @@ func (v *Volume) garbageLevel() float64 {
 
 func (v *Volume) Compact() error {
 	glog.V(3).Infof("Compacting ...")
-	v.accessLock.Lock()
-	defer v.accessLock.Unlock()
-	glog.V(3).Infof("Got Compaction lock...")
+	//no need to lock for copy on write
+	//v.accessLock.Lock()
+	//defer v.accessLock.Unlock()
+	//glog.V(3).Infof("Got Compaction lock...")
 
 	filePath := v.FileName()
 	glog.V(3).Infof("creating copies for volume %d ...", v.Id)
@@ -59,10 +60,15 @@ func (v *Volume) copyDataAndGenerateIndexFile(dstName, idxName string) (err erro
 	nm := NewNeedleMap(idx)
 	new_offset := int64(SuperBlockSize)
 
+	now := uint64(time.Now().Unix())
+
 	err = ScanVolumeFile(v.dir, v.Collection, v.Id, func(superBlock SuperBlock) error {
 		_, err = dst.Write(superBlock.Bytes())
 		return err
 	}, true, func(n *Needle, offset int64) error {
+		if n.HasTtl() && now >= n.LastModified+uint64(v.Ttl.Minutes()*60) {
+			return nil
+		}
 		nv, ok := v.nm.Get(n.Id)
 		glog.V(4).Infoln("needle expected offset ", offset, "ok", ok, "nv", nv)
 		if ok && int64(nv.Offset)*NeedlePaddingSize == offset && nv.Size > 0 {