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. - 

std::vector<std::future<Coordination::ExistsResponse>> exists_futures;
    std::vector<std::future<Coordination::MultiResponse>> remove_futures;
    exists_futures.reserve(part_names.size());
    remove_futures.reserve(part_names.size());
    try
    {
        
        
        for (const String & part_name : part_names)
        {
            String part_path = fs::path(replica_path) / "parts" / part_name;
            exists_futures.emplace_back(zookeeper->asyncExists(part_path));
        }
        for (size_t i = 0; i < part_names.size(); ++i)
        {
            Coordination::ExistsResponse exists_resp = exists_futures[i].get();
            if (exists_resp.error == Coordination::Error::ZOK)
            {
                Coordination::Requests ops;
                removePartFromZooKeeper(part_names[i], ops, exists_resp.stat.numChildren > 0);
                remove_futures.emplace_back(zookeeper->tryAsyncMulti(ops));
            }
            else
            {
                LOG_DEBUG(log, "There is no part {} in ZooKeeper, it was only in filesystem", part_names[i]);
                
                remove_futures.emplace_back();
            }
        }
    }