Figure out join() and Drop behavior for DownloadHandle

[graebm]

Currently, DownloadHandle has a join() function, but neither cp.rs or the benchmark runner bother to call it. It was added because UploadHandle has a join(), and we thought the APIs should be similar.

But DownloadHandle::join() cancels the download, while UploadHandle::join() waits for the upload to successfully complete, and has a separate UploadHandle::abort() function. Should we rename DownloadHandle::join() to DownloadHandle::abort()?

Also, what's the Drop behavior for DownloadHandle? We should probably ensure its spawned tasks get cancelled ASAP. Likewise for UploadHandle/UploadObjectsHandle/DownloadObjectsHandle

• Drop and cancel for UploadHandle/UploadObjectsHandle (PR)
• Drop and cancel for DownloadHandle/DownloadObjectsHandle