fs: cleanup - do not cache size of metafiles (#3630)

* Remove Size() method and size field from lock.LockedFile
* WriteTo method of fsMeta and uploadsV1 now takes concrete type *lock.LockedFile

pkg/lock/lock_test.go

@@ -80,9 +80,6 @@ func TestRWLockedFile(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if rlk.Size() != 0 {
-		t.Fatal("File size should be zero", rlk.Size())
-	}
 	isClosed := rlk.IsClosed()
 	if isClosed {
 		t.Fatal("File ref count shouldn't be zero")