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.

obscache: warm the cache in all cases There are case when the obsstore have been invalidated, but we still need to update the cache.

obscache: update the format to allow negative tiprev If the changelog is empty (but the obsstore is not) the 'tiprev' will be -1.

obscache: load the disk data before checking is the cache is up to date This is actually required since non-loaded cache will never be up to date...

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 So outdated cache mean another client have been mudding the repository but things will get back in place at the next transaction.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.

readme: mention that some improvement are enabled for 4.2 only