Provide a means to check http connection equality for streaming connections.

If one uses an http::Pipe::Writer to stream a response, one cannot compare the writer with another to see if the connection has changed.

This is useful for example, in the master's http api when there is asynchronous disconnection logic. When we handle the disconnection, it's possible for the scheduler to have re-subscribed, and so the master needs to tell if the disconnection event is relevant for the current connection before taking action.