LUCENE-4396: BooleanScorer should sometimes be used for MUST clauses

by Da Huang for Apache Software Foundation

I aim to improve the performance of BooleanQuery by implementing a better rule for choosing Scorer. In order to support the new rule, my proposed work also includes a new mechanism for BooleanScorer to support MUST clauses friendly, and improved implementation of BooleanScorer which makes it able to act as Sub-Scorer on some conditions. I'll also compare the performance between my new rule and the old one.