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. - 

}
        Coordination::Responses results;
        Coordination::Error rc = zookeeper->tryMulti(ops, results);
        
        
        
        partition_block_numbers_holder.reset();
        if (rc == Coordination::Error::ZOK)
        {
            if (have_mutation)
            {
                
                String alter_path = dynamic_cast<const Coordination::CreateResponse &>(*results[alter_path_idx]).path_created;
                alter_entry->znode_name = alter_path.substr(alter_path.find_last_of('/') + 1);
                
                String mutation_path = dynamic_cast<const Coordination::CreateResponse &>(*results[mutation_path_idx]).path_created;
                mutation_znode = mutation_path.substr(mutation_path.find_last_of('/') + 1);
            }
            else
            {
                
                String alter_path = dynamic_cast<const Coordination::CreateResponse &>(*results[alter_path_idx]).path_created;
                alter_entry->znode_name = alter_path.substr(alter_path.find_last_of('/') + 1);
            }
            break;
        }
        else if (rc == Coordination::Error::ZBADVERSION)
        {
            if (results[0]->error != Coordination::Error::ZOK)
                throw Exception("Metadata on replica is not up to date with common metadata in Zookeeper. Cannot alter",