Only emit key changes which are different from what we had before (#1279)

We did this already for local `/keys/upload` but didn't for
remote `/users/devices`. This meant any resyncs would spam produce
events, hammering disk i/o and spamming the logs.

keyserver/internal/device_list_update_test.go

@@ -91,6 +91,10 @@ func (d *mockDeviceListUpdaterDatabase) PrevIDsExists(ctx context.Context, userI
 	return d.prevIDsExist(userID, prevIDs), nil
 }
 
+func (d *mockDeviceListUpdaterDatabase) DeviceKeysJSON(ctx context.Context, keys []api.DeviceMessage) error {
+	return nil
+}
+
 type roundTripper struct {
 	fn func(*http.Request) (*http.Response, error)
 }