* s3: populate Initiated timestamp in ListMultipartUploads
ListMultipartUploads returned each upload with only Key and UploadId,
omitting the Initiated timestamp. Clients such as GeeseFS rely on this
field to expire stale uploads and crash on its absence. Set Initiated
from the upload directory entry creation time so repeated listings
preserve the original initiation time.
* test/s3: verify Initiated timestamp in ListMultipartUploads
Add an integration test that initiates a multipart upload, lists it,
and asserts the Initiated field is populated and preserved across
repeated listings rather than reflecting the listing time.