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 (shutdown_called)
        return;
    shutdown_called = true;
    
    {
        std::lock_guard lock(mutation_wait_mutex);
        mutation_wait_event.notify_all();
    }
    merger_mutator.merges_blocker.cancelForever();
    parts_mover.moves_blocker.cancelForever();
    background_executor.finish();
    background_moves_executor.finish();
    try
    {
        
        
        
        
        
        clearOldPartsFromFilesystem(true);
    }
    catch (...)
    {
        
        
        tryLogCurrentException(log);
    }
}
StorageMergeTree::~StorageMergeTree()