SOLR-5734: We should use System.nanoTime rather than System.currentTimeMillis when calculating elapsed time. (merged branch_4x r1573426)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_7@1581162 13f79535-47bb-0310-9956-ffa450edef68