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. - 

{
    auto zookeeper = getZooKeeper();
    Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
    
    std::shuffle(replicas.begin(), replicas.end(), thread_local_rng);
    for (const String & replica : replicas)
    {
        if (replica == replica_name)
            continue;
        if (active && !zookeeper->exists(fs::path(zookeeper_path) / "replicas" / replica / "is_active"))
            continue;
        String largest_part_found;
        Strings parts = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas" / replica / "parts");
        for (const String & part_on_replica : parts)
        {
            if (part_on_replica == entry.new_part_name
                || MergeTreePartInfo::contains(part_on_replica, entry.new_part_name, format_version))
            {
                if (largest_part_found.empty()