http: add Flusher type; remove Flush from ResponseWriter

http: add Flusher type; remove Flush from ResponseWriter

The Flush functionality wasn't removed, but now you have
to test if your ResponseWriter is also a Flusher:

func ServeHTTP(rw http.ResponseWriter, req *http.Request) {
   if f, ok := rw.(http.Flusher); ok {
       f.Flush()
   }
}

R=rsc, bradfitzwork
CC=gburd, golang-dev
https://golang.org/cl/4239077