Create a cache backend that pre loads multiple items in one getMultiple() call

Problem/Motivation

Another approach to solve the same problems that https://www.drupal.org/node/2348245 → tried to tackle. We have things like views that have twelve plugin managers (and six handler managers) Whenever we use them we are getting the cached definitions individually for each manager, when we always need almost all of them for a view to work.

I think more import here is making less calls to the cache, rather than shaving lots of time off the page load. If a site comes under a lot of load, lots of more calls to your cache/database etc.. is not good news.

Proposed resolution

Create a backend that all plugin managers can use that collects cids for each plugin definition and loads them all in one getMultiple() call. Subsequent calls to get() individual definitions will then fetch it from the already loaded data.

Remaining tasks

Is this concept ligit? tests, reviews, settle on plan - all the things.

User interface changes

None

API changes

None