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 (!allow_renaming)
        throw Exception("Cannot rename Replicated table, because zookeeper_path contains implicit 'database' or 'table' macro. "
                        "We cannot rename path in ZooKeeper, so path may become inconsistent with table name. If you really want to rename table, "
                        "you should edit metadata file first and restart server or reattach the table.", ErrorCodes::NOT_IMPLEMENTED);
}
void StorageReplicatedMergeTree::rename(const String & new_path_to_table_data, const StorageID & new_table_id)
{
    checkTableCanBeRenamed();
    MergeTreeData::rename(new_path_to_table_data, new_table_id);
    
    if (!is_readonly)
    {
        
        
        try
        {
            auto zookeeper = getZooKeeper();
            zookeeper->set(fs::path(replica_path) / "host", getReplicatedMergeTreeAddress().toString());
        }
        catch (Coordination::Exception & e)
        {
            LOG_WARNING(log, "Cannot update the value of 'host' node (replica address) in ZooKeeper: {}", e.displayText());
        }
    }
    
}
bool StorageReplicatedMergeTree::existsNodeCached(const std::string & path) const