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. - 

for (auto disk_it = disks.rbegin(); disk_it != disks.rend(); ++disk_it)
    {
        auto disk_ptr = *disk_it;
        for (auto it = disk_ptr->iterateDirectory(relative_data_path); it->isValid(); it->next())
        {
            
            if (startsWith(it->name(), "tmp") || it->name() == MergeTreeData::FORMAT_VERSION_FILE_NAME || it->name() == MergeTreeData::DETACHED_DIR_NAME)
                continue;
            if (!startsWith(it->name(), MergeTreeWriteAheadLog::WAL_FILE_NAME))
                part_names_with_disks.emplace_back(it->name(), disk_ptr);
            else if (it->name() == MergeTreeWriteAheadLog::DEFAULT_WAL_FILE_NAME && settings->in_memory_parts_enable_wal)
            {
                
                write_ahead_log = std::make_shared<MergeTreeWriteAheadLog>(*this, disk_ptr, it->name());
                for (auto && part : write_ahead_log->restore(metadata_snapshot, getContext()))
                    parts_from_wal.push_back(std::move(part));
            }
            else if (settings->in_memory_parts_enable_wal)
            {
                MergeTreeWriteAheadLog wal(*this, disk_ptr, it->name());
                for (auto && part : wal.restore(metadata_snapshot, getContext()))
                    parts_from_wal.push_back(std::move(part));
            }
        }
    }
    auto part_lock = lockParts();
    data_parts_indexes.clear();
    if (part_names_with_disks.empty() && parts_from_wal.empty())
    {
        LOG_DEBUG(log, "There are no data parts");