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.
Ah right, yes. So when the `tbb_hasher` stuff is happening inside the concurrent_unordered_map, it's able to fall back on `operator==` to resolve collisions - that's not possible if we do the hashing externally...