Make book navigation block cacheable

Problem/Motivation

From BookNavigationBlock:

  /**
   * {@inheritdoc}
   *
   * @todo Make cacheable as part of https://drupal.org/node/1805054
   */
  public function getCacheMaxAge() {
    return 0;
  }

Proposed resolution

Once #1805054: Cache localized, access filtered, URL resolved, and rendered menu trees lands, doing this becomes possible.

But, it doesn't make sense to fix it as part of that issue, because it requires updating quite a (non-public) code in BookManager. Sadly, BookManager basically reimplements a lot of the menu tree building/manipulating handling. If it'd use the menu tree building instead, then we'd get it for free.

Remaining tasks

See #4.

User interface changes

None.

API changes

TBD