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();
    
    String quorum_status_path = fs::path(zookeeper_path) / "quorum" / "status";
    if (is_parallel)
        quorum_status_path = fs::path(zookeeper_path) / "quorum" / "parallel" / part_name;
    
    const String quorum_last_part_path = fs::path(zookeeper_path) / "quorum" / "last_part";
    String value;
    Coordination::Stat stat;
    
    while (zookeeper->tryGet(quorum_status_path, value, &stat))
    {
        ReplicatedMergeTreeQuorumEntry quorum_entry(value);
        if (quorum_entry.part_name != part_name)
        {
            LOG_TRACE(log, "Quorum {}, already achieved for part {} current part {}",
                      quorum_status_path, part_name, quorum_entry.part_name);
            
            break;
        }
        quorum_entry.replicas.insert(replica_name);
        if (quorum_entry.replicas.size() >= quorum_entry.required_number_of_replicas)
        {
            
            LOG_TRACE(log, "Got {} replicas confirmed quorum {}, going to remove node",
                      quorum_entry.replicas.size(), quorum_status_path);
            Coordination::Requests ops;
            Coordination::Responses responses;