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, I see - neat! So much better than that nonsense I was spouting. So we'd do something like manually call tbb_hasher( Key ) to get a size_t and then internally use a concurrent_unordered_map<size_t, Value>, basically with an identity hasher?

I would still prefer to treat that as a separate issue though if that's OK - this stuff has been quite tricky so I'd like to roll out changes incrementally for testing rather than lots all at once.