HDFS-10528. Add logging to successful standby checkpointing. Contributed by Xiaoyu Yao.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyCheckpointer.java

@@ -439,6 +439,7 @@ public class StandbyCheckpointer {
               namesystem.setNeedRollbackFsImage(false);
             }
             lastCheckpointTime = now;
+            LOG.info("Checkpoint finished successfully.");
           }
         } catch (SaveNamespaceCancelledException ce) {
           LOG.info("Checkpoint was cancelled: " + ce.getMessage());