Make sure the Query API exposes IndexReader(s) in some way

Referring to the mailing list thread "Grouping lucene query on Infinispan", since we're planning to remove method org.infinispan.query.SearchManager.getSearchFactory(), and alternative should be provider at least to access IndexReader.

Note that the plan to remove this had good reasons: to abstract the user from the indexes and so make sure we could have different types of underlying indexes.. so this would need to be exposed at best in form of "advanced API", and clarify the restrictions.