Split the persistent cache option into logical parts.

Presently setting the persistent cache option has two effects:

1. The cache instance is held onto and future requests for it get given the original instance.
2. Information passing through the cache is held onto in a static var for performance serving of subsequent requests.

It's become apparent that really we need two options here.
For example Fred's changes on MDL-13114.
It makes no sense to use the static var for request caches - and doing so adds both processing and memory overhead.

This change needs to be made and immediately applied to MDL-13114