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 lock = lockParts();
    for (const auto & part : remove)
    {
        auto it_part = data_parts_by_info.find(part->info);
        if (it_part == data_parts_by_info.end())
            throw Exception("Part " + part->getNameWithState() + " not found in data_parts", ErrorCodes::LOGICAL_ERROR);
        modifyPartState(part, IMergeTreeDataPart::State::Temporary);
        
        data_parts_indexes.erase(it_part);
    }
}
void MergeTreeData::removePartsFromWorkingSet(const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock * acquired_lock)