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. - 

{
    
    fetcher.blocker.cancelForever();
    merger_mutator.merges_blocker.cancelForever();
    parts_mover.moves_blocker.cancelForever();
    restarting_thread.shutdown();
    background_executor.finish();
    part_moves_between_shards_orchestrator.shutdown();
    {
        auto lock = queue.lockQueue();
        
        
        
        queue.pull_log_blocker.cancelForever();
    }
    background_moves_executor.finish();
    auto data_parts_exchange_ptr = std::atomic_exchange(&data_parts_exchange_endpoint, InterserverIOEndpointPtr{});
    if (data_parts_exchange_ptr)
    {
        getContext()->getInterserverIOHandler().removeEndpointIfExists(data_parts_exchange_ptr->getId(replica_path));
        
        data_parts_exchange_ptr->blocker.cancelForever();