cbbackupmgr should persist the s3 upload manifest as soon as it's created

When the multipart uploader is first created, we create a new S3 upload manifest; it's important that we persist this upload manifest as soon as it has been created. This is because this file is used to abort invalid mutlipart uploads on a resume; if this file doesn't exist yet it's possible for cbbacukpmgr to try to resume the DCP stream from the wrong seqno.