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. - 

drop_range.partition_id = partition_id;
        drop_range.min_block = 0;
        drop_range.max_block = increment.get(); 
        drop_range.level = std::numeric_limits<decltype(drop_range.level)>::max();
    }
    
    try
    {
        {
            
            
            Transaction transaction(*this);
            auto data_parts_lock = lockParts();
            
            for (MutableDataPartPtr & part : dst_parts)
                renameTempPartAndReplace(part, &increment, &transaction, data_parts_lock);
            transaction.commit(&data_parts_lock);
            
            if (replace)
                removePartsInRangeFromWorkingSet(drop_range, true, data_parts_lock);
        }
        PartLog::addNewParts(getContext(), dst_parts, watch.elapsed());
    }
    catch (...)
    {
        PartLog::addNewParts(getContext(), dst_parts, watch.elapsed(), ExecutionStatus::fromCurrentException());
        throw;
    }
}
void StorageMergeTree::movePartitionToTable(const StoragePtr & dest_table, const ASTPtr & partition, ContextPtr local_context)