obscache: skip the cache entirely if not up to date
The current update code has some race condition windows around updating. But we
also ensure the cache are up to date with every transaction. So outdated cache
mean another client have been mudding the repository but things will get back
in place at the next transaction.
So we just skip using the cache when not up to date. This is not the best we
could do. But this is good enough for now.