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. - 

continue;
        source_queue.push_back(entry);
    }
    
    cloneMetadataIfNeeded(source_replica, source_path, zookeeper);
    
    Strings source_replica_parts = zookeeper->getChildren(fs::path(source_path) / "parts");
    ActiveDataPartSet active_parts_set(format_version, source_replica_parts);
    Strings active_parts = active_parts_set.getParts();
    
    Strings local_parts_in_zk = zookeeper->getChildren(fs::path(replica_path) / "parts");
    Strings parts_to_remove_from_zk;
    for (const auto & part : local_parts_in_zk)
    {
        if (active_parts_set.getContainingPart(part).empty())
        {
            queue.remove(zookeeper, part);
            parts_to_remove_from_zk.emplace_back(part);
            LOG_WARNING(log, "Source replica does not have part {}. Removing it from ZooKeeper.", part);
        }
    }
    tryRemovePartsFromZooKeeperWithRetries(parts_to_remove_from_zk);
    auto local_active_parts = getDataParts();
    DataPartsVector parts_to_remove_from_working_set;
    for (const auto & part : local_active_parts)
    {
        if (active_parts_set.getContainingPart(part->name).empty())
        {
            parts_to_remove_from_working_set.emplace_back(part);
            LOG_WARNING(log, "Source replica does not have part {}. Removing it from working set.", part->name);