ZOOKEEPER-4057: NIOServerCnxnFactory.selectorThreads,using List instead of HashSet

[lanicc]

Using List costs less memory and computing than HashSet.

[maoling]

@lanicc

Using List costs less memory and computing than HashSet.

Could you please give us more explanations for this? Any benchmark report?

[lanicc]

simple test:

`public static void main(String[] args) {
int numObj = 1;

    Set<Object> set = new HashSet<>();
    for (int i = 0; i < numObj; i++) {
        set.add(new Object());
    }
    System.out.println("Size of HashSet: " + GraphLayout.parseInstance(set).totalSize());
    List<Object> list = new ArrayList<>();
    for (int i = 0; i < numObj; i++) {
        list.add(new Object());
    }
    System.out.println("Size of ArrayList: " + GraphLayout.parseInstance(list).totalSize());


    long start, end;

    Iterator<Object> setIterator = set.iterator();
    start = System.nanoTime();
    while (setIterator.hasNext()) {
        setIterator.next();
    }
    end = System.nanoTime();
    System.out.println("HashSet iterator cost: " + (end - start));

    Iterator<Object> listIterator = list.iterator();
    start = System.nanoTime();
    while (listIterator.hasNext()) {
        listIterator.next();
    }
    end = System.nanoTime();
    System.out.println("ArrayList iterator cost: " + (end - start));
}`

result:

Size of HashSet: 256
Size of ArrayList: 112
HashSet iterator cost: 20474
ArrayList iterator cost: 460

[lanicc]

@maoling