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/filer/directory_in_map.go

@@ -187,10 +187,10 @@ func (dm *DirectoryManagerInMap) makeDirectory(dirPath string) (dir *DirectoryEn
 	for i := 1; i < len(parts); i++ {
 		sub, ok := dir.SubDirectories[parts[i]]
 		if !ok {
-		  var err error
+			var err error
 			sub, err = dm.NewDirectoryEntryInMap(dir, parts[i])
 			if err != nil {
-			  return nil, false
+				return nil, false
 			}
 			dir.SubDirectories[parts[i]] = sub
 			created = true