This PR is the final change, where all backends are now purely implemented with async interfaces.
The code changes here replace the LIST call with an async version. This affect all backends (and then some) but the changes are mostly mechanical rewrites of the code.
In many places, the backeds were already prepared for async output, in other some glue was needed.
To reduce the scope of this change, some backends simply report a synchronous result as an async list.
Without await, the using statement can dispose the Stream before the
call to PutAsync completes, resulting in an ObjectDisposedException.
This fixes#4556.
Kenneth Skovhede