initial commit of the aggregations module

Closes #3300
force default codec
reduce p/c searches and num child docs being generated
Upgrade to Lucene 4.6

This commit upgrades to Lucene 4.6 and contains the following improvements:

 * Remove XIndexWriter in favor of the fixed IndexWriter
 * Removes patched XLuceneConstantScoreQuery
 * Now uses Lucene passage formatters contributed from Elasticsearch in PostingsHighlighter
 * Upgrades to Lucene46 Codec from Lucene45 Codec
 * Fixes problem in CommonTermsQueryParser where close was never called.