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. - 

const DataPartPtr & part)
{
    auto zookeeper = getZooKeeper();
    while (true)
    {
        Coordination::Requests ops;
        NameSet absent_part_paths_on_replicas;
        
        checkPartChecksumsAndAddCommitOps(zookeeper, part, ops, part->name, &absent_part_paths_on_replicas);
        
        if (transaction.isEmpty())
            return {};
        
        if (!absent_part_paths_on_replicas.empty())
        {
            Coordination::Requests new_ops;
            for (const String & part_path : absent_part_paths_on_replicas)
            {
                new_ops.emplace_back(zkutil::makeCreateRequest(part_path, "", zkutil::CreateMode::Persistent));