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. - 

bool supportsFinal() const override
    {
        return merging_params.mode == MergingParams::Collapsing
            || merging_params.mode == MergingParams::Summing
            || merging_params.mode == MergingParams::Aggregating
            || merging_params.mode == MergingParams::Replacing
            || merging_params.mode == MergingParams::VersionedCollapsing;
    }
    bool supportsSubcolumns() const override { return true; }