SOLR-1880: Distributed Search skips GET_FIELDS stage if EXECUTE_QUERY stage gets all fields. Requests with fl=id or fl=id,score are now single-pass.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1571153 13f79535-47bb-0310-9956-ffa450edef68