obscache: set a valid "empty" cache key if the cache is missing This avoid detecting bad cache when it just happens to be no cache.

obscache: have the obsstore fix empty file cachekey Before this change, the missing file and empty file returned different value.

obscache: log case where the cache is not up to date This will help track performance issue

obscache: still update and use the cache during transaction Other code might access the obsolete set in the middle of a transaction. It is better to use the case in that case since the update will eventually be written when the transaction is committed.