WriteChunk copied the whole chunk, which lib/multipart already hands over
in a buffer from the global memory pool, into a bytes.Buffer so that
retries could re-send it. That doubled the per-part memory and made a
fresh chunk-sized heap allocation (64 MiB by default) for every part,
times the upload concurrency.
The chunk reader is seekable, so find its size with Seek and rewind it
inside the pacer closure instead, sending the pooled buffer directly.
Also fix the error for a part that fails to upload, which formatted the
buffer instead of the part number.
Nick Craig-Wood
337f762c79