Part 1: 2301239, while correcting tag to tags, clarify the wording about cache bins and tags.
Part 1: 2301239, clean up related to MenuLinkDefault, cast return, docs summary to third person, clarify array of what.
2301239 Fixed @todo about plugin definition service
Part 1: 2301239 Created follow up issue to increase performance for getRootPathIds.
Part 1: follow up issue to increase performance by finding out if menus have expanded links.
Part 1: 2301239 Removed @todo about hashing, removed hash SHA from menu_cid.
Part 1: 2301239 Follow up issue to check if menu_parent_extend_child index is being used.
START PART 2:  https://www.drupal.org/node/2301273
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkTree-review-do-not-test.patch
START PART 3: https://www.drupal.org/node/2301313
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkContentUI-review-do-not-test.patch