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. - 

* Otherwise there is race condition - merge of blocks could happen in interval that doesn't yet contain new part.
      */
    if (increment)
    {
        part_info.min_block = part_info.max_block = increment->get();
        part_info.mutation = 0; 
        part_name = part->getNewName(part_info);
    }
    else 
        part_name = part->name;
    LOG_TRACE(log, "Renaming temporary part {} to {}.", part->relative_path, part_name);
    auto it_duplicate = data_parts_by_info.find(part_info);
    if (it_duplicate != data_parts_by_info.end())
    {
        String message = "Part " + (*it_duplicate)->getNameWithState() + " already exists";
        if ((*it_duplicate)->checkState({DataPartState::Outdated, DataPartState::Deleting}))
            throw Exception(message + ", but it will be deleted soon", ErrorCodes::PART_IS_TEMPORARILY_LOCKED);
        throw Exception(message, ErrorCodes::DUPLICATE_DATA_PART);
    }
    DataPartPtr covering_part;
    DataPartsVector covered_parts = getActivePartsToReplace(part_info, part_name, covering_part, lock);
    DataPartsVector covered_parts_in_memory;
    if (covering_part)
    {
        LOG_WARNING(log, "Tried to add obsolete part {} covered by {}", part_name, covering_part->getNameWithState());
        return false;
    }