Async-all-the-way: Consider making IOperationProcessor and IDocumentProcessor Async

[dgioulakis]

This would be a breaking change but one easy to change and update existing processors.

The pipeline is already async-ready:

public async Task<SwaggerDocument> GenerateForControllersAsync(IEnumerable<Type> controllerTypes)
        {
            var document = await CreateDocumentAsync(Settings).ConfigureAwait(false);
            var schemaResolver = new SwaggerSchemaResolver(document, Settings);

            foreach (var controllerType in controllerTypes)
                await GenerateForControllerAsync(document, controllerType, new SwaggerGenerator(_schemaGenerator, Settings, schemaResolver)).ConfigureAwait(false);

            document.GenerateOperationIds();

            foreach (var processor in Settings.DocumentProcessors)
                processor.Process(new DocumentProcessorContext(document, controllerTypes, schemaResolver, _schemaGenerator));

            return document;
        }

[RicoSuter]

We should definitely do that. The question is whether to break or support backward comp. (ie two interfaces, processors as list of object).