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. - 

assert(prev_ptr == nullptr);
        getContext()->getInterserverIOHandler().addEndpoint(data_parts_exchange_ptr->getId(replica_path), data_parts_exchange_ptr);
        
        restarting_thread.start();
        
        startup_event.wait();
        
        
        background_executor.start();
        startBackgroundMovesIfNeeded();
        part_moves_between_shards_orchestrator.start();
    }
    catch (...)
    {
        
        
        
        try
        {
            shutdown();
        }
        catch (...)
        {
            std::terminate();
        }
        
        throw;
    }
}
void StorageReplicatedMergeTree::shutdown()