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. - 

{
            throw Exception(
                "Can not clone replica, because the " + source_replica + " updated to new ClickHouse version",
                ErrorCodes::REPLICA_STATUS_CHANGED);
        }
        else if (responses[1]->error == Coordination::Error::ZBADVERSION)
        {
            
            
            throw Exception(
                "Can not clone replica, because the " + source_replica + " became lost", ErrorCodes::REPLICA_STATUS_CHANGED);
        }
        else if (responses.back()->error == Coordination::Error::ZBADVERSION)
        {
            
            
            LOG_WARNING(log, "Log pointer of source replica {} changed while we loading queue nodes. Will retry.", source_replica);
            continue;
        }
        else
        {
            zkutil::KeeperMultiException::check(rc, ops, responses);
        }
    }
    std::sort(source_queue_names.begin(), source_queue_names.end());
    Strings source_queue;
    for (const String & entry_name : source_queue_names)
    {
        String entry;
        if (!zookeeper->tryGet(fs::path(source_path) / "queue" / entry_name, entry))