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. - 

{
    auto remove_part = [&](const auto & part)
    {
        LOG_DEBUG(log, "Removing part from filesystem {}", part.name);
        try
        {
            bool keep_s3 = !this->unlockSharedData(part);
            part.remove(keep_s3);
        }
        catch (...)
        {
            tryLogCurrentException(log, "There is a problem with deleting part " + part.name + " from filesystem");
        }
    };
    const auto settings = getSettings();
    if (settings->max_part_removal_threads > 1 && parts.size() > settings->concurrent_part_removal_threshold)
    {
        
        size_t num_threads = std::min<size_t>(settings->max_part_removal_threads, parts.size());
        ThreadPool pool(num_threads);
        
        for (const DataPartPtr & part : parts)
        {
            pool.scheduleOrThrowOnError([&, thread_group = CurrentThread::getGroup()]
            {
                SCOPE_EXIT_SAFE(
                    if (thread_group)
                        CurrentThread::detachQueryIfNotDetached();