Simplify ExactPhraseScorer

While looking at this scorer i see a few little things which are remnants of the past:

• crazy heuristics to use next() over advance(): I think it should just use advance(), like conjunctionscorer. these days advance() isnt stupid anymore
• incorrect leapfrogging. the lead scorer is never advanced if a subsequent scorer goes past it, it just falls into this nextDoc() loop.
• pre-next()'ing: we are using cost() api to sort, so there is no need to do that.
• UnionDocsAndPositionsEnum doesnt follow docsenum contract and set initial doc to -1
• postingsreader advance() doesnt need to check docFreq > BLOCK_SIZE on each advance call, thats easy to remove.

So I think really this scorer should just look like "conjunctionscorer that verifies positions on match".