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 (!candidates.empty())
        {
            /
            std::shuffle(candidates.begin(), candidates.end(), thread_local_rng);
            String selected_disk_name = candidates.front();
            WriteBufferFromOwnString log_str;
            writeCString("\nbalancer: \n", log_str);
            for (const auto & [disk_name, per_disk_parts] : disk_parts_for_logging)
                writeString(fmt::format("  {}: [{}]\n", disk_name, fmt::join(per_disk_parts, ", ")), log_str);
            LOG_DEBUG(log, log_str.str());
            if (ttl_infos)
                reserved_space = tryReserveSpacePreferringTTLRules(
                    metadata_snapshot,
                    part_size,
                    *ttl_infos,
                    time(nullptr),
                    max_volume_index,
                    is_insert,
                    getStoragePolicy()->getDiskByName(selected_disk_name));
            else
                reserved_space = tryReserveSpace(part_size, getStoragePolicy()->getDiskByName(selected_disk_name));
            if (reserved_space)
            {
                currently_emerging_big_parts.emplace(
                    part_name, EmergingPartInfo{reserved_space->getDisk(0)->getName(), part_info.partition_id, part_size});
                for (const auto & part : covered_parts)
                {
                    if (currently_submerging_big_parts.count(part))