added "now := time.Now()"

mvcc/backend/backend.go

@@ -291,6 +291,8 @@ func (b *backend) Defrag() error {
 }
 
 func (b *backend) defrag() error {
+	now := time.Now()
+	
 	// TODO: make this non-blocking?
 	// lock batchTx to ensure nobody is using previous tx, and then
 	// close previous ongoing tx.