Add some way to close an ongoing HTTPRequest (AsyncHTTPClient)

[enki]

just call _finish on the handle?

i have two use cases: one is to restart a streaming connection, the other is to limit the amount of data i want to read (not interested in downloading gigabytes)

[bdarnell]

I don't know if calling _finish() will shut it down cleanly. Closing the socket will definitely work, although there's probably a more graceful way to do it. We don't currently expose any clean way to get at this information. AsyncHTTPClient.fetch() should probably return an object with a cancel() method.