Before this patch there was:

- parallel_distributed_insert_select=1, that executes:

    INSERT INTO dist_out SELECT ... FROM underlying_dist_in

After this patch there will be:

- parallel_distributed_insert_select=2, that executes:

    INSERT INTO underlying_dist_out SELECT ... FROM underlying_dist_in

And cover the behaviour w/o integration test, by using the following
techincs:
- SYSTEM STOP DISTRIBUTED SENDS
- prefer_localhost_replica=0