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. - 

current_zookeeper = getContext()->getAuxiliaryZooKeeper(zookeeper_name);
            }
        }
        catch (...)
        {
            if (!attach)
                dropIfEmpty();
            throw;
        }
    }
    bool skip_sanity_checks = false;
    if (current_zookeeper && current_zookeeper->exists(replica_path + "/flags/force_restore_data"))
    {
        skip_sanity_checks = true;
        current_zookeeper->remove(replica_path + "/flags/force_restore_data");
        LOG_WARNING(log, "Skipping the limits on severity of changes to data parts and columns (flag {}/flags/force_restore_data).", replica_path);
    }
    else if (has_force_restore_data_flag)
    {
        skip_sanity_checks = true;
        LOG_WARNING(log, "Skipping the limits on severity of changes to data parts and columns (flag force_restore_data).");
    }
    loadDataParts(skip_sanity_checks);
    if (!current_zookeeper)
    {
        if (!attach)
        {
            dropIfEmpty();
            throw Exception("Can't create replicated table without ZooKeeper", ErrorCodes::NO_ZOOKEEPER);
        }