Simplify LRU Cache

Reading into the backport of the LRU cache suggests that other users have also found it to be slow.  It is apparently aimed at uses that are highly IO bound and require robust cache key management - it does not work well for caching methods that are already "reasonably fast".  

This pull request introduces a few small changes that help with this.  

1. Remove the cache info.  Rez is not using it, and amazing for such a small amount of code gives a 5-10% improvement in performance.
2. Simplify the `_make_key` method based on a implementation found in the Python bug tracker (linked to in module docstring).  This reduces the time it takes to make a key by ~50% and is sufficient for our needs.