LRUCache optimisation

This improves LRUCache performance pretty substantially (particularly for the multithreaded case) while keeping the same basic algorithm as it had before. I'm afraid the bulk of the work is in a single commit, because the rather wandering path I took to get to this point didn't make sense as individual commits. I have however added a lot of documentation to the private members which hopefully does a pretty decent job of explaining what's going on - so it probably makes more sense just to read the final result rather than look at the diffs.
I've noticed that the erase method will lock the list and then here we call the removal callback. So in other words, the removal callback will be limited as to what it can do with the LRUCache object. Should we document it in the class definition?