* s3: keep empty-folder cleanup out of the multipart .uploads staging tree
The async EmptyFolderCleaner deleted <bucket>/.uploads once it looked
empty. A concurrent CreateMultipartUpload inserting its marker between
the cleaner's emptiness check and the bulk child delete had its row
wiped, so the upload silently vanished: ListMultipartUploads then omits
it and the following part/complete/copy calls fail. Skip the .uploads
subtree in both the queue and the delete path (including the eager
parent cascade); the multipart upload lifecycle owns it.
* s3: slice the bucket-relative path instead of reallocating it
Chris Lu
b43089f721