Fix CacheItem pointer in cache

We had an inconsistency with CacheItem vs *CacheItem in the LRU cache, causing gitlab-ce tests to fail. The reason the tests didn't pick this up is because I only had benchmarks exercising the cache code, not tests--so those benchmarks didn't run.

in gitlab-ce, since all feature flags are on by default the cache was exercised and it caused the panic

I added a couple of tests that exercises the cache code in an actual test