Don't include auth credentials in debug log

Patch by Sam Tunnicliffe; reviewed by Brandon Williams for
CASSANDRA-9682

CHANGES.txt

@@ -1,4 +1,5 @@
 2.0.17
+ * Don't include auth credentials in debug log (CASSANDRA-9682)
  * Can't transition from write survey to normal mode (CASSANDRA-9740)
  * Avoid NPE in AuthSuccess#decode (CASSANDRA-9727)
  * Add listen_address to system.local (CASSANDRA-9603)

src/java/org/apache/cassandra/transport/messages/CredentialsMessage.java

@@ -89,6 +89,6 @@ public class CredentialsMessage extends Message.Request
     @Override
     public String toString()
     {
-        return "CREDENTIALS " + credentials;
+        return "CREDENTIALS";
     }
 }