By: Tom Sydney Kerckhove <syd@cs-syd.eu>
server: record NARs in objects table on multipart completion A NAR whose multipart upload completes but whose closure never commits (a sibling object in the push batch failed, or the push was killed) stays absent from the objects table, so every later closure re-opens a multipart upload for the already-present NAR. The client then finds it present and skips it, leaking an orphaned zero-part multipart upload each run. Record the object as soon as its multipart completes. Fixes #446