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. - 

{
    LOG_INFO(log, "Waiting mutation: {}", file_name);
    {
        auto check = [version, this]()
        {
            if (shutdown_called)
                return true;
            auto mutation_status = getIncompleteMutationsStatus(version);
            return !mutation_status || mutation_status->is_done || !mutation_status->latest_fail_reason.empty();
        };
        std::unique_lock lock(mutation_wait_mutex);
        mutation_wait_event.wait(lock, check);
    }
    
    std::set<String> mutation_ids;
    mutation_ids.insert(file_name);
    auto mutation_status = getIncompleteMutationsStatus(version, &mutation_ids);
    try
    {
        checkMutationStatus(mutation_status, mutation_ids);
    }
    catch (...)
    {
        tryLogCurrentException(__PRETTY_FUNCTION__);
        throw;
    }
    LOG_INFO(log, "Mutation {} done", file_name);
}
void StorageMergeTree::mutate(const MutationCommands & commands, ContextPtr query_context)