irmin-pack: improve control file's atomic write properties

Currently, we assume that writes are atomic because the control file is small enough to fit within a page. We could improve this by using a known mechanism for atomic updates, like writing to a temporary file and renaming it.

Context https://github.com/mirage/irmin/pull/2180/files/7b9e971b221b577c26dd533f9f8853bf77967325#r1090672123