Rename `Fut` into `ResponseFuture`

[prasannavl]

Currently, the associated params of HttpService are ConnectionFuture which corresponds to connect, and the Fut which corresponds to respond.

I think it'd be better to call this ResponseFuture such that it maintains parity.

connect -> ConnectionFuture
respond -> ResponseFuture

rather than the Fut that isn't really informative or intuitive.

[yoshuawuyts]

I've struggled with this before, and think these changes would indeed add more clarity 👍.

[prasannavl]

PR in :)