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. - 

{
            /
            if (committed_big_parts_from_partition.find(name) == committed_big_parts_from_partition.end()
                && part_info.partition_id == emerging_part.partition_id)
            {
                auto it = disk_occupation.find(emerging_part.disk_name);
                if (it != disk_occupation.end())
                {
                    it->second += emerging_part.estimate_bytes;
                    disk_parts_for_logging[emerging_part.disk_name].push_back(
                        formatReadableSizeWithBinarySuffix(emerging_part.estimate_bytes) + " (emerging)");
                }
                else
                {
                    /
                }
            }
        }
        size_t min_occupation_size = std::numeric_limits<size_t>::max();
        std::vector<String> candidates;
        for (const auto & [disk_name, size] : disk_occupation)
        {
            if (size < min_occupation_size)
            {
                min_occupation_size = size;
                candidates = {disk_name};
            }
            else if (size == min_occupation_size)
            {
                candidates.push_back(disk_name);