The channel-based algorithm had grown quite complicated. This is easier
to reason about and likely to be more performant with very many
CompleteBlob calls.
When backing up many small files, the unbuffered channels frequently
cause the FileSaver to block when reporting progress information. Thus,
add buffers to these channels to avoid unnecessary scheduling.
As the status information is purely informational, it doesn't matter
that the status reporting shutdown is somewhat racy and could miss a few
final updates.
