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. - 

ReplicatedMergeTreeMutationEntry mutation_entry;
    mutation_entry.source_replica = replica_name;
    mutation_entry.commands = commands;
    const String mutations_path = fs::path(zookeeper_path) / "mutations";
    const auto zookeeper = getZooKeeper();
    
    
    
    while (true)
    {
        Coordination::Stat mutations_stat;
        zookeeper->get(mutations_path, &mutations_stat);
        PartitionBlockNumbersHolder partition_block_numbers_holder =
                allocateBlockNumbersInAffectedPartitions(mutation_entry.commands, query_context, zookeeper);
        mutation_entry.block_numbers = partition_block_numbers_holder.getBlockNumbers();
        mutation_entry.create_time = time(nullptr);
        
        
        
        Coordination::Requests requests;
        requests.emplace_back(zkutil::makeSetRequest(mutations_path, String(), mutations_stat.version));
        requests.emplace_back(zkutil::makeCreateRequest(
            fs::path(mutations_path) / "", mutation_entry.toString(), zkutil::CreateMode::PersistentSequential));
        if (auto txn = query_context->getZooKeeperMetadataTransaction())
            txn->moveOpsTo(requests);
        Coordination::Responses responses;
        Coordination::Error rc = zookeeper->tryMulti(requests, responses);
        partition_block_numbers_holder.reset();
        if (rc == Coordination::Error::ZOK)
        {