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/operation/assign_file_id.go

@@ -17,7 +17,7 @@ type AssignResult struct {
 	Error     string `json:"error,omitempty"`
 }
 
-func Assign(server string, count int, replication string, collection string) (*AssignResult, error) {
+func Assign(server string, count int, replication string, collection string, ttl string) (*AssignResult, error) {
 	values := make(url.Values)
 	values.Add("count", strconv.Itoa(count))
 	if replication != "" {
@@ -26,6 +26,9 @@ func Assign(server string, count int, replication string, collection string) (*A
 	if collection != "" {
 		values.Add("collection", collection)
 	}
+	if ttl != "" {
+		values.Add("ttl", ttl)
+	}
 	jsonBlob, err := util.Post("http://"+server+"/dir/assign", values)
 	glog.V(2).Info("assign result :", string(jsonBlob))
 	if err != nil {