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. - 

{
    DataPartsVector res;
    
    std::unique_lock lock(grab_old_parts_mutex, std::defer_lock);
    if (!lock.try_lock())
        return res;
    time_t now = time(nullptr);
    std::vector<DataPartIteratorByStateAndInfo> parts_to_delete;
    {
        auto parts_lock = lockParts();
        auto outdated_parts_range = getDataPartsStateRange(DataPartState::Outdated);
        for (auto it = outdated_parts_range.begin(); it != outdated_parts_range.end(); ++it)
        {
            const DataPartPtr & part = *it;
            auto part_remove_time = part->remove_time.load(std::memory_order_relaxed);
            if (part.unique() && 
                ((part_remove_time < now &&
                now - part_remove_time > getSettings()->old_parts_lifetime.totalSeconds()) || force
                || isInMemoryPart(part))) 
            {
                parts_to_delete.emplace_back(it);
            }
        }
        res.reserve(parts_to_delete.size());
        for (const auto & it_to_delete : parts_to_delete)
        {
            res.emplace_back(*it_to_delete);
            modifyPartState(it_to_delete, DataPartState::Deleting);
        }
    }