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. - 

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            Ordinary            = 0,    
            Collapsing          = 1,
            Summing             = 2,
            Aggregating         = 3,
            Replacing           = 5,
            Graphite            = 6,
            VersionedCollapsing = 7,
        };
        Mode mode;
        
        String sign_column;
        
        Names columns_to_sum;
        
        String version_column;
        
        Graphite::Params graphite_params;
        
        void check(const StorageInMemoryMetadata & metadata) const;
        String getModeName() const;
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