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. - 

{
    if (!storage.is_leader)
        return false;
    auto zk = storage.getZooKeeper();
    std::optional<Entry> entry_to_process;
    
    {
        std::lock_guard lock(state_mutex);
        for (auto const & entry : entries | boost::adaptors::map_values)
        {
            if (entry.state.value == EntryState::DONE || entry.state.value == EntryState::CANCELLED)
                continue;
            entry_to_process.emplace(entry);
            break;
        }
    }
    if (!entry_to_process.has_value())
        return false;
    
    
    
    
    
    zkutil::EphemeralNodeHolder::Ptr entry_node_holder;
    try