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. - 

getStorageID().getFullTableName() + " (StorageReplicatedMergeTree::mutationsUpdatingTask)", [this]{ mutationsUpdatingTask(); });
    merge_selecting_task = getContext()->getSchedulePool().createTask(
        getStorageID().getFullTableName() + " (StorageReplicatedMergeTree::mergeSelectingTask)", [this] { mergeSelectingTask(); });
    
    merge_selecting_task->deactivate();
    mutations_finalizing_task = getContext()->getSchedulePool().createTask(
        getStorageID().getFullTableName() + " (StorageReplicatedMergeTree::mutationsFinalizingTask)", [this] { mutationsFinalizingTask(); });
    if (getContext()->hasZooKeeper() || getContext()->hasAuxiliaryZooKeeper(zookeeper_name))
    {
        
        
        
        
        
        
        
        
        
        
        
        
        
        try
        {
            if (zookeeper_name == default_zookeeper_name)
            {
                current_zookeeper = getContext()->getZooKeeper();
            }
            else
            {