When enqueuing references check whether the j.l.r.Reference has already been enqeued or removed in the lock. Do not enqueue them again. This occurs because multiple threads may try to enqueue the same j.l.r.Reference at the same time.
Reviewed-by: mchung, dholmes, plevart, shade
6711667: (ref) Update SoftReference timestamp only if clock advances
Forward port from 6u14; originally fixed by Tom Rodriguez in earlier update
Reviewed-by: martin
