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.
This introduces a few constants in favor of multiple strings with the same value.
The supported arguments in the module have been augmented to better report possible values and default values.
The options parser has been updated to work better with CLI where the options are not default filled.
Kenneth Skovhede