obsolete: remove last instance of _enabled The _enabled bool has been replaced by obsolete.isenabled(...). This removes the last instance of it so I can remove the _enabled flag entirely shortly.

revbranchcache: write cache even during read operations Previously we would only actually write the revbranchcache to disk if we were in the middle of a write operation (like commit). Now we will also write it during any read operation. The cache knows how to invalidate itself, so it shouldn't become corrupt if multiple writers try at once (and the write-on-read behavior/risk is the same as all our other caches).