LUCENE-5675: pull out FieldReader from BTTR

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5675@1595006 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5675: pull out FieldReader from BTTR

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5675@1595007 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5675: pull out IntersectEnum

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5675@1595013 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5675: fix javadocs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5675@1595017 13f79535-47bb-0310-9956-ffa450edef68