UnifiedHighlighter: internally visit the query tree once

Ideally the UnifiedHighlighter should "visit" the query tree once instead of several times (weight.extractTerms, MultiTermHighlighting, PhraseHelper). Perhaps this can happen in one new class, perhaps called QueryExtractor. It's debatable wether this would replace a bunch of fields presently on UHComponents or whether it would simply help produce the existing UHComponents; shrug.

Admittedly, I don't know how much of an "optimization" this is, or wether this is just a refactoring that is done on principle. I simply like the principle of it; knowing there are multiple _visit_s to the query gnaws at me.