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. - 

{
    return f.version < s.version;
}
}
std::optional<MergeTreeMutationStatus> StorageMergeTree::getIncompleteMutationsStatus(Int64 mutation_version, std::set<String> * mutation_ids) const
{
    std::lock_guard lock(currently_processing_in_background_mutex);
    auto current_mutation_it = current_mutations_by_version.find(mutation_version);
    
    if (current_mutation_it == current_mutations_by_version.end())
        return {};
    MergeTreeMutationStatus result{.is_done = false};
    const auto & mutation_entry = current_mutation_it->second;
    auto data_parts = getDataPartsVector();
    for (const auto & data_part : data_parts)
    {
        if (data_part->info.getDataVersion() < mutation_version)
        {
            if (!mutation_entry.latest_fail_reason.empty())
            {
                result.latest_failed_part = mutation_entry.latest_failed_part;
                result.latest_fail_reason = mutation_entry.latest_fail_reason;
                result.latest_fail_time = mutation_entry.latest_fail_time;