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.
By changing to IEnumerable, it is possible to iterate only a portion of the list, which is useful when not all entries are needed (e.g., when testing a connection).
All existing backends have been updated, and any which were able to be changed to yield return results in a straightforward way now do.
Many backends had a try/catch in the List() method. Due to the fact that yield returns can't be placed within a try/catch block, these have been refactored to either scope the try/catch to the parts that (should) be the only places throwing exceptions, so that exceptions are still caught and handled.
Note that lazy evaluation may cause some changes in behavior - exceptions that were previously thrown at the point of invokation of List() may now be thrown while it is being enumerated.
I believe this will not be problematic though, as the only well-known exception seems to be FolderMissingException, which should be thrown by Test(), but TestList() attempts to enumerate the list to force this exception.
Any places that require the legacy behavior can get it by simply converting the lazy enumerable to a List()
Kenneth Skovhede