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. - 

dummy_alter.source_replica = source_replica;
    dummy_alter.metadata_str = source_metadata;
    dummy_alter.columns_str = source_columns;
    dummy_alter.alter_version = source_metadata_version;
    dummy_alter.create_time = time(nullptr);
    zookeeper->create(replica_path + "/queue/queue-", dummy_alter.toString(), zkutil::CreateMode::PersistentSequential);
    
    
    
}
void StorageReplicatedMergeTree::cloneReplicaIfNeeded(zkutil::ZooKeeperPtr zookeeper)
{
    Coordination::Stat is_lost_stat;
    bool is_new_replica = true;
    String res;
    if (zookeeper->tryGet(fs::path(replica_path) / "is_lost", res, &is_lost_stat))
    {
        if (res == "0")
            return;
        if (is_lost_stat.version)
            is_new_replica = false;
    }
    else
    {
        
        
        
        zookeeper->create(fs::path(replica_path) / "is_lost", "0", zkutil::CreateMode::Persistent);
        return;
    }