obshashrange: rework saving branching to be clearer and safer

sqlcache: protect read query too Some error (like locked database) can even happens when doing readonly operation. So we protect them too. At that point, it seems that pysqlite3 is not the right tool for this job.

sqlite: fast path when nothing to save If we have no new data to save, we should try to do anything. Doing something concurrently is hard enough. If we can avoid doing it at all that would be great.