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. - 

{
    const auto settings = getSettings();
    if (parts_to_remove.size() > 1 && settings->max_part_removal_threads > 1 && parts_to_remove.size() > settings->concurrent_part_removal_threshold)
    {
        
        size_t num_threads = std::min<size_t>(settings->max_part_removal_threads, parts_to_remove.size());
        ThreadPool pool(num_threads);
        
        for (const DataPartPtr & part : parts_to_remove)
        {
            pool.scheduleOrThrowOnError([&, thread_group = CurrentThread::getGroup()]
            {
                SCOPE_EXIT_SAFE(
                    if (thread_group)
                        CurrentThread::detachQueryIfNotDetached();
                );
                if (thread_group)
                    CurrentThread::attachTo(thread_group);
                LOG_DEBUG(log, "Removing part from filesystem {}", part->name);
                part->remove();
            });