cmd/devp2p: be very correct about route53 change splitting (#20820)

Turns out the way RDATA limits work is documented after all,
I just didn't search right. The trick to make it work is to
count UPSERTs twice.

This also adds an additional check to ensure TTL changes are
applied on existing records.

cmd/devp2p/dns_route53_test.go

@@ -140,11 +140,23 @@ func TestRoute53ChangeSort(t *testing.T) {
 		t.Fatalf("wrong changes (got %d, want %d)", len(changes), len(wantChanges))
 	}
 
+	// Check splitting according to size.
 	wantSplit := [][]*route53.Change{
 		wantChanges[:4],
-		wantChanges[4:8],
+		wantChanges[4:6],
+		wantChanges[6:],
 	}
-	split := splitChanges(changes, 600)
+	split := splitChanges(changes, 600, 4000)
+	if !reflect.DeepEqual(split, wantSplit) {
+		t.Fatalf("wrong split batches: got %d, want %d", len(split), len(wantSplit))
+	}
+
+	// Check splitting according to count.
+	wantSplit = [][]*route53.Change{
+		wantChanges[:5],
+		wantChanges[5:],
+	}
+	split = splitChanges(changes, 10000, 6)
 	if !reflect.DeepEqual(split, wantSplit) {
 		t.Fatalf("wrong split batches: got %d, want %d", len(split), len(wantSplit))
 	}