This factors copy out of SetModTime and Copy so it can be called from
both places.
This also reworks all the multipart uploading to use sync.Errgroup and
memory pooling like the other backends. This makes it more memory
efficient and handle errors better.
See: https://forum.rclone.org/t/copying-files-within-a-b2-bucket/16680/10
Before this fix rclone would just use the authorised bucket regardless
of what bucket you put on the command line.
This uses the new `bucketName` response in the API and checks that the
user is using the correct bucket name to avoid accidents.
Fixes#2839
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
