[meta] Speed up routing related code

Problem/Motivation

The routing subsystem does a couple of stuff:

From an incoming request, it determines the active controller, which is used to render the "current page".
On that page, there might be links to other elements of the page, so it generates URLs, to those places.

On top of that people enter in PATHs in menu links etc. for which we have to determine the routing information as well, but potentially
multiple times per request.

Each of those 3 usecases have dedicated issues to speed things up:

1. #2406117: potentially cache parts of routing #2084675: Quering the database for router items is slow (access checking)
2. #1965074: Add cache wrapper to the UrlGenerator #2074297: Optimize the code in doGenerate() in the UrlGenerator to take advantage of Drupal path restrictions.
3. #2370651: Make (routing by path) cacheable

Proposed resolution

Remaining tasks

User interface changes

API changes