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. - 

{
    if (zookeeper->expired())
        throw Exception("Table was not dropped because ZooKeeper session has expired.", ErrorCodes::TABLE_WAS_NOT_DROPPED);
    auto remote_replica_path = zookeeper_path + "/replicas/" + replica;
    LOG_INFO(logger, "Removing replica {}, marking it as lost", remote_replica_path);
    
    
    zookeeper->trySet(zookeeper_path + "/replicas/" + replica + "/is_lost", "1");
    
    zookeeper->tryRemoveRecursive(remote_replica_path);
    if (zookeeper->exists(remote_replica_path))
        LOG_ERROR(logger, "Replica was not completely removed from ZooKeeper, {} still exists and may contain some garbage.", remote_replica_path);
    
    Strings replicas;
    if (Coordination::Error::ZOK != zookeeper->tryGetChildren(zookeeper_path + "/replicas", replicas) || !replicas.empty())
        return;
    LOG_INFO(logger, "{} is the last replica, will remove table", remote_replica_path);
    /** At this moment, another replica can be created and we cannot remove the table.
      * Try to remove /replicas node first. If we successfully removed it,
      * it guarantees that we are the only replica that proceed to remove the table
      * and no new replicas can be created after that moment (it requires the existence of /replicas node).
      * and table cannot be recreated with new /replicas node on another servers while we are removing data,
      * because table creation is executed in single transaction that will conflict with remaining nodes.
      */