HBASE-11028 FSLog: Avoid an extra sync if the current transaction is already sync'd

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1589158 13f79535-47bb-0310-9956-ffa450edef68
HBASE-11000 Add autoflush option to PerformanceEvaluation - addendum

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1589162 13f79535-47bb-0310-9956-ffa450edef68