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. - 

}
            else
                throw Coordination::Exception(code, quorum_status_path);
        }
        else
        {
            LOG_TRACE(log, "Quorum {} still not satisfied (have only {} replicas), updating node",
                      quorum_status_path, quorum_entry.replicas.size());
            
            auto code = zookeeper->trySet(quorum_status_path, quorum_entry.toString(), stat.version);
            if (code == Coordination::Error::ZOK)
            {
                break;
            }
            else if (code == Coordination::Error::ZNONODE)
            {
                
                break;
            }
            else if (code == Coordination::Error::ZBADVERSION)
            {
                
                continue;
            }
            else
                throw Coordination::Exception(code, quorum_status_path);
        }
    }
}
void StorageReplicatedMergeTree::cleanLastPartNode(const String & partition_id)