Async microgateway example and test

[pointlander]

In theory we support the async execution of a microgateway action:
https://github.com/project-flogo/microgateway/blob/master/metadata.go#L7
https://github.com/project-flogo/microgateway/blob/master/internal/core/core.go#L78

Create an example and test for the async execution of a microgateway action. The microgateway action can be as simple as having a single call to the log service/activity. Fix any bugs related to the async execution of a microgateway action.