Rewrite CalcMemoized using FutureTask

[skapral]

Subj. This would simplify the solution's maintenance.

Prototype.

class Util {
    public static <T> T memoizedCalculation(ConcurrentHashMap<Key, FutureTask<T>> cache, Key key, Callable<T> calculation) {
        try {
            FutureTask<T> t = cache.computeIfAbsent(key, k -> new FutureTask<>(calculation));
            t.run();
            return t.get();
        } catch(Exception ex) {
            throw new RuntimeException(ex);
        }
    }
}