Improvments to BasicCompletes necessary by vlingo-net-http implementation

[tjaskula]

• Passing in onActionCallback to continuation so if the inner completes camputes the result it will call back to the passed in function (avoids threads locking or spining)
• Checking completes if the inner completes has a results before registering a new continuation
• Before running registered continuation we check if next can be executed based on the previous continuation result
• Otherwise continuation sets known outcome