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.