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. - 

{
    switch (entry.state.value)
    {
        case EntryState::DONE:
            break;
        case EntryState::CANCELLED:
            break;
        case EntryState::TODO:
        {
            
            Entry entry_copy = entry;
            entry_copy.state = EntryState::SYNC_SOURCE;
            entry_copy.update_time = std::time(nullptr);
            zk->set(entry_copy.znode_path, entry_copy.toString(), entry_copy.version);
        }