Make async version of ProcessController exposed and used in aiida-core #315
Description
In #297, the ProcessController interface is set only for the synchronous process controller, but not fit for the async process controller.
The reason is that in aiida-core only the sync one RemoteProcessThreadController is used. In aiida-core this part is not exposed to the public as user space API by hiding behind the CLI. Then why not use the async process controller?