I think we should simply make a hard break on the Sim.lengthNorm ->
computeNorm cutover. Subclassing sim is an expert thing, and, I'd
rather apps see a compilation error on upgrade so that they realize
their lengthNorm wasn't being called this whole time because of
LUCENE-2828 (and that they must now cutover to computeNorm).
So I made lengthNorm final (and throws UOE), computeNorm abstract. I
deprecated SimilarityDelegator, and fixed BQ to not use it anymore.
The only other use is FuzzyLikeThisQuery, but fixing that is a little
too involved for today.