accesslib: mark_context_dirty() - fix race condition

accesslib: mark_context_dirty() - fix race condition

We had a 1s race condition where a user could get their rights loaded
at the exact time an admin is changing roles/caps and see the "old"
data. Or even see a half-updated view of the access controls.

Yuck.

So we fix the race condition backdating the dirtyness. Cheap, but
effective. And then we backdate it some more to cover for minor clock
flutter on clusters (you still need ntp however!).