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 mutation_status = queue.getIncompleteMutationsStatus(mutation_id);
            
            if (mutation_status && !mutation_status->latest_fail_reason.empty())
                break;
        }
        
        
        if (!getZooKeeper()->exists(fs::path(zookeeper_path) / "mutations" / mutation_id))
        {
            throw Exception(ErrorCodes::UNFINISHED, "Mutation {} was killed, manually removed or table was dropped", mutation_id);
        }
        
        std::set<String> mutation_ids;
        mutation_ids.insert(mutation_id);
        
        
        auto mutation_status = queue.getIncompleteMutationsStatus(mutation_id, &mutation_ids);
        checkMutationStatus(mutation_status, mutation_ids);
        if (partial_shutdown_called)
            throw Exception("Mutation is not finished because table shutdown was called. It will be done after table restart.",
                ErrorCodes::UNFINISHED);
    }
    if (!inactive_replicas.empty())
    {
        throw Exception(ErrorCodes::UNFINISHED,
                        "Mutation is not finished because some replicas are inactive right now: {}. Mutation will be done asynchronously",
                        boost::algorithm::join(inactive_replicas, ", "));
    }
}
void StorageReplicatedMergeTree::createNewZooKeeperNodes()