Primary cache: don't denormalize auto-generated instance keys

[teh-cmc]

The primary cache currently treats all components the same way, it's completely unaware of semantics.

A side-effect of this is that it will denormalize instance keys just like it will denormalize any other component: i.e. it stores the result of applying all the join, implicit and splat logic.
That means that if you have 1 million implicit instance keys, they will actually be materialized in the cache and take ~7.6MiB of space.