rbc: fix invalid rbc-revs entries caused by missing cache growth It was in some cases possible to end up writing to the cache file without growing it first. The range assignment in _setcachedata would append instead of writing at the requested position and thus write the new record in the wrong place. To fix this, we avoid looking up in too small caches, and when growing the cache, do it right before writing the new record to it so we know it has been done correctly.