Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

{
    if (!storage.getSettings()->part_moves_between_shards_enable)
        return;
    if (need_stop)
        return;
    auto sleep_ms = 10;
    try
    {
        fetchStateFromZK();
        if (step())
            fetchStateFromZK();
        else
            sleep_ms = 3 * 1000;
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
    }
    task->scheduleAfter(sleep_ms);
}
void PartMovesBetweenShardsOrchestrator::shutdown()