Code review request: 8011124: Make KerberosTime immutable

[Weijun Wang]

http://cr.openjdk.java.net/~weijun/8011124/webrev.00/
KerberosTime is a very basic data type and it is nice to be immutable.

One test is updated. No new regression test for this code change. 
Considered noreg-cleanup.

Thanks
Max