Change default Sim to BM25 (w/backcompat config handling)

LUCENE-6789 changed the default similarity for IndexSearcher to BM25 and renamed "DefaultSimilarity" to "ClassicSimilarity"

Solr needs to be updated accordingly:

• a "ClassicSimilarityFactory" should exist w/expected behavior/javadocs
• default behavior (in 6.0) when no similarity is specified in configs should (ultimately) use BM25 depending on luceneMatchVersion
  • either by assuming BM25SimilarityFactory or by changing the internal behavior of DefaultSimilarityFactory
• comments in sample configs need updated to reflect new default behavior
• ref guide needs updated anywhere it mentions/implies that a particular similarity is used (or implies TF-IDF is used by default)