BREAK CHANGE: HttpConnect.close removed, and handlers shall return Future instead

[Tom Yeh]

In the upcoming Rikulo Stream 0.7, the interface between the server and the handlers are changed to simplify the chaining of request handling and error handling:

1. The request handler must return `Future` if it spawned an asynchronous task.
   
2. The request filter must return `Future`.
   
3. The request handler can't return a forwarding URI. Rather, it shall invoke and return `connect.forward(uri)` instead.
   
4. The request handler needs not to close the connection. It is done automatically. In fact, there is no way to close it manually.
   
5. RSP will import `dart:async` by default.
6. The `close`, `onClose` and `onError` methods of `HttpConnect` are removed. Chaining request handlers is straightforward: it is the same as chaining `Future` objects.
   

Remember to read the Request Handling section  again when 0.7 is released. There will be more detailed description.