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. - 

{
    need_stop = true;
    task->deactivate();
    LOG_TRACE(log, "PartMovesBetweenShardsOrchestrator thread finished");
}
void PartMovesBetweenShardsOrchestrator::fetchStateFromZK()
{
    std::lock_guard lock(state_mutex);
    entries.clear();
    auto zk = storage.getZooKeeper();
    Strings task_names = zk->getChildren(entries_znode_path);
    for (auto const & task_name : task_names)
    {
        PartMovesBetweenShardsOrchestrator::Entry e;
        Coordination::Stat stat;
        e.znode_path = entries_znode_path + "/" + task_name;
        auto entry_str = zk->get(e.znode_path, &stat);
        e.fromString(entry_str);
        e.version = stat.version;
        e.znode_name = task_name;
        entries[task_name] = std::move(e);
    }
}
bool PartMovesBetweenShardsOrchestrator::step()