Optimize two calls of io.Copy which cannot make use of neither
io.ReaderFrom nor io.WriterTo optimization tricks by replacing them with
io.CopyBuffer with reusable buffers.

First is fallback call to io.Copy when server misses the optimized case
of using sendfile to copy from a regular file to net.TCPConn; second is
use of io.Copy on piped reader/writer when handler implementation uses
http.CloseNotifier interface. One of the notable users of
http.CloseNotifier is httputil.ReverseProxy.

benchmark                    old ns/op     new ns/op     delta
BenchmarkCloseNotifier-4     309591        303388        -2.00%

benchmark                    old allocs     new allocs     delta
BenchmarkCloseNotifier-4     50             49             -2.00%

benchmark                    old bytes     new bytes     delta
BenchmarkCloseNotifier-4     36168         3140          -91.32%

Fixes #12455

Change-Id: I512e6aa2f1aeed2ed00246afb3350c819b65b87e
Reviewed-on: https://go-review.googlesource.com/14177
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>