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. - 

if (deduplication_log)
    {
        String block_id = part->getZeroLevelPartBlockID();
        auto res = deduplication_log->addPart(block_id, part_info);
        if (!res.second)
        {
            ProfileEvents::increment(ProfileEvents::DuplicatedInsertedBlocks);
            LOG_INFO(log, "Block with ID {} already exists as part {}; ignoring it", block_id, res.first.getPartName());
            return false;
        }
    }
    
    
    
    
    part->name = part_name;
    part->info = part_info;
    part->is_temp = false;
    part->setState(DataPartState::PreCommitted);
    part->renameTo(part_name, true);
    auto part_it = data_parts_indexes.insert(part).first;
    if (out_transaction)
    {
        out_transaction->precommitted_parts.insert(part);
    }
    else
    {
        size_t reduce_bytes = 0;
        size_t reduce_rows = 0;