Try to retrieve individual blobs via LoadBlob if streaming did not work.

Due to the interface of streamPack, we cannot guarantee that operations
progress fast enough that the underlying connections remains open. This
introduces partial failures which massively complicate the error
handling.

Switch to a simpler approach that retrieves the pack in chunks of 32MB.
If a blob is larger than this limit, then it is downloaded separately.

To avoid multiple copies in memory, an auxiliary interface
`discardReader` is introduced that allows directly accessing the
downloaded byte slices, while still supporting the streaming used by the
`check` command.

With most cloud providers, traffic is much more expensive than API
calls. Thus slightly bias streamPack towards a bit more API calls in
exchange for slightly less traffic.