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. - 

{
        entry_node_holder = zkutil::EphemeralNodeHolder::create(entry_to_process->znode_path + "/lock_holder", *zk, storage.replica_name);
    }
    catch (const Coordination::Exception & e)
    {
        if (e.code == Coordination::Error::ZNODEEXISTS)
        {
            LOG_DEBUG(log, "Task {} is being processed by another replica", entry_to_process->znode_name);
            return false;
        }
        throw;
    }
    try
    {
        
        
        stepEntry(entry_to_process.value(), zk);
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        Entry entry_copy = entry_to_process.value();
        entry_copy.last_exception_msg = getCurrentExceptionMessage(false);
        entry_copy.update_time = std::time(nullptr);
        zk->set(entry_copy.znode_path, entry_copy.toString(), entry_copy.version);
        return false;
    }
    return true;
}
void PartMovesBetweenShardsOrchestrator::stepEntry(const Entry & entry, zkutil::ZooKeeperPtr zk)