obscache: update the cache key in place Instead of computing the key in place, we update the existing one using the data used for the incremental update of the content. This will help reaching purely incremental cache eventually. The 'getcachekey' function is dropped as it is no longer used.

obscache: distinct 'clear' and 'reset' We make a difference between basic invalidatiton (or lack of loading) and 'reset' (current cache data are invalid). We can now ensure a non-None key when we start the update. This will allow us to "update" the existing key instead of recomputing one from scratch. This is more efficient and less racy.

merge with stable