Load menu links for multiple routes

Problem/Motivation

This is a follow-up issue to #1578832: [already commited but followup for CR?] <front> menu links are missing active trail classes.

Fixing the other issue introduced a performance penalty for the front page because an extra query had to be run in the menu system to check for link items matching <front>. This could be avoided if we had the ability to load menu links for multiple route names instead of just one at a time. Per @larowlan:

I don't think we can avoid the extra queries because we only have \Drupal\Core\Menu\MenuLinkManager::loadLinksByRoute and \Drupal\Core\Menu\MenuTreeStorageInterface::loadByRoute but I think it would be worth adding loadLinksByRoutes and loadByRoutes methods to both those services to support doing this in a single query.

@catch agreed that this task could be done in a follow-up issue.

Proposed resolution

• Add \Drupal\Core\Menu\MenuLinkManager::loadLinksByRoutes.
• Add \Drupal\Core\Menu\MenuTreeStorageInterface::loadByRoutes.
• Update \Drupal\Core\Menu\MenuActiveTrail::getActiveLink() to use loadLinksByRoutes() for better performance.

Remaining tasks

User interface changes

Introduced terminology

API changes

Two new functions, one in MenuLinkManager and another in MenuTreeStorageInterface.

Data model changes

Release notes snippet