ZOOKEEPER-4029: improve the log of FileTxnLog#append method

Author: chenghuaqing <chenghuaqing123@126.com>

Reviewers: maoling <maoling@apache.org>

Closes #1694 from seuCHENGHQ/ZOOKEEPER-4029

zookeeper-server/src/main/java/org/apache/zookeeper/server/persistence/FileTxnLog.java

@@ -43,6 +43,7 @@ import org.apache.jute.BinaryOutputArchive;
 import org.apache.jute.InputArchive;
 import org.apache.jute.OutputArchive;
 import org.apache.jute.Record;
+import org.apache.zookeeper.server.Request;
 import org.apache.zookeeper.server.ServerMetrics;
 import org.apache.zookeeper.server.ServerStats;
 import org.apache.zookeeper.server.TxnLogEntry;
@@ -276,7 +277,7 @@ public class FileTxnLog implements TxnLog, Closeable {
                 "Current zxid {} is <= {} for {}",
                 hdr.getZxid(),
                 lastZxidSeen,
-                hdr.getType());
+                Request.op2String(hdr.getType()));
         } else {
             lastZxidSeen = hdr.getZxid();
         }