Pierre-Yves David <pierre-yves.david@octobus.net> [Tue, 09 May 2017 19:02:04 +0200] rev 2333
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.