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_time = clear_without_timeout ? 0 : time(nullptr);
    for (const DataPartPtr & part : remove)
    {
        if (part->getState() == IMergeTreeDataPart::State::Committed)
        {
            removePartContributionToColumnSizes(part);
            removePartContributionToDataVolume(part);
        }
        if (part->getState() == IMergeTreeDataPart::State::Committed || clear_without_timeout)
            part->remove_time.store(remove_time, std::memory_order_relaxed);
        if (part->getState() != IMergeTreeDataPart::State::Outdated)
            modifyPartState(part, IMergeTreeDataPart::State::Outdated);
        if (isInMemoryPart(part) && getSettings()->in_memory_parts_enable_wal)
            getWriteAheadLog()->dropPart(part->name);
    }
}
void MergeTreeData::removePartsFromWorkingSetImmediatelyAndSetTemporaryState(const DataPartsVector & remove)