This change includes removing older azureblob storage SDK, and getting
parity to existing code with latest blob storage SDK.
This change is also pre-req for addressing #2091
In a typical rclone copy to a bucket/container based remote, before
this change we were doing a list, followed by a HEAD of the bucket to
check it existed before doing the copy. The fact the list succeeded
means the bucket exists so mark it OK at that point.
Issue #1421
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.