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 MergeTreePartInfo & part_info, MergeTreeData::DataPartState state, DataPartsLock & ) const
{
    auto current_state_parts_range = getDataPartsStateRange(state);
    
    auto it = data_parts_by_state_and_info.lower_bound(DataPartStateAndInfo{state, part_info});
    if (it != current_state_parts_range.end())
    {
        if ((*it)->info == part_info)
            return *it;
        if ((*it)->info.contains(part_info))
            return *it;
    }
    if (it != current_state_parts_range.begin())
    {
        --it;
        if ((*it)->info.contains(part_info))
            return *it;
    }
    return nullptr;
}
void MergeTreeData::swapActivePart(MergeTreeData::DataPartPtr part_copy)