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. - 

const PartVersionWithName needle{mutation_version, ""};
        auto versions_it = std::lower_bound(
            part_versions_with_names.begin(), part_versions_with_names.end(), needle, comparator);
        size_t parts_to_do = versions_it - part_versions_with_names.begin();
        Names parts_to_do_names;
        parts_to_do_names.reserve(parts_to_do);
        for (size_t i = 0; i < parts_to_do; ++i)
            parts_to_do_names.push_back(part_versions_with_names[i].name);
        std::map<String, Int64> block_numbers_map({{"", entry.block_number}});
        for (const MutationCommand & command : entry.commands)
        {
            WriteBufferFromOwnString buf;
            formatAST(*command.ast, buf, false, true);
            result.push_back(MergeTreeMutationStatus
            {
                entry.file_name,
                buf.str(),
                entry.create_time,
                block_numbers_map,
                parts_to_do_names,
                parts_to_do_names.empty(),
                entry.latest_failed_part,
                entry.latest_fail_time,
                entry.latest_fail_reason,
            });
        }
    }
    return result;
}
CancellationCode StorageMergeTree::killMutation(const String & mutation_id)