Before this change when NewObject was called the b2 backend would list
the directory that the object was in in order to find it.
Unfortunately list calls are Class C transactions and cost more.
This patch switches to using HEAD requests instead. These are Class B
transactions. It is then necessary to parse the headers from response
back into the data that we get from the listing. However B2 returns
exactly the same data, just in a different form.
Rclone will use the old directory listing method when looking for
files with versions as these can't be found via a HEAD request.
This change will particularly benefit --files-from, rclone serve
restic but most operations will see some benefit.
cb16f42075