HBASE-20047 AuthenticationTokenIdentifier should provide a toString

Signed-off-by: Chia-Ping Tsai <chia7712@gmail.com>

hbase-client/src/main/java/org/apache/hadoop/hbase/security/token/AuthenticationTokenIdentifier.java

@@ -187,4 +187,11 @@ public class AuthenticationTokenIdentifier extends TokenIdentifier {
   public int hashCode() {
     return (int)sequenceNumber;
   }
+
+  @Override
+  public String toString() {
+    return "(username=" + username + ", keyId="
+            + keyId + ", issueDate=" + issueDate
+            + ", expirationDate=" + expirationDate + ", sequenceNumber=" + sequenceNumber + ")";
+  }
 }