LUCENE-5207, LUCENE-5376: add expressions support to lucene server, so you can define a virtual field from any JS expression and then sort by that field or retrieve its values for all hits

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5376@1556508 13f79535-47bb-0310-9956-ffa450edef68