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. - 

clearOldWriteAheadLogs();
    clearEmptyParts();
    
    
    clearOldTemporaryDirectories(0);
    
    time_after_previous_cleanup.restart();
    try
    {
        background_executor.start();
        startBackgroundMovesIfNeeded();
    }
    catch (...)
    {
        
        
        
        try
        {
            shutdown();
        }
        catch (...)
        {
            std::terminate();
        }
        
        throw;
    }
}
void StorageMergeTree::shutdown()