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. - 

Poco::Event startup_event;
    
    std::atomic<bool> partial_shutdown_called {false};
    
    Poco::Event partial_shutdown_event {false};     
    
    static std::atomic_uint total_fetches;
    
    std::atomic_uint current_table_fetches {0};
    int metadata_version = 0;
    
    BackgroundJobsExecutor background_executor;
    BackgroundMovesExecutor background_moves_executor;
    
    bool queue_update_in_progress = false;
    BackgroundSchedulePool::TaskHolder queue_updating_task;
    BackgroundSchedulePool::TaskHolder mutations_updating_task;
    
    BackgroundSchedulePool::TaskHolder merge_selecting_task;
    
    std::mutex merge_selecting_mutex;
    
    BackgroundSchedulePool::TaskHolder mutations_finalizing_task;
    
    ReplicatedMergeTreeCleanupThread cleanup_thread;
    
    ReplicatedMergeTreePartCheckThread part_check_thread;
    
    ReplicatedMergeTreeRestartingThread restarting_thread;