net/http/httptrace: add retry hook

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.7.4 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/benburkert"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.4/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.4/libexec/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/qs/qkt9twmx4qg379d6f8kxl1vm0000gn/T/go-build714318584=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

https://play.golang.org/p/odjH-4qcwa

What did you expect to see?

The WroteRequest hook is called only once for the second request, as it is with the first request, and the program exits normally.

What did you see instead?

The WroteRequest hook is called twice because the transport attempts to write the request to a dead connection then retries with a new connection. Both attempts call the hook. This causes the program to panic because of the send on the already closed channel during the second call.

panic: send on closed channel

goroutine 12 [running]:
panic(0x3167c0, 0x1070a648)
	/usr/local/go/src/runtime/panic.go:500 +0x720
main.request.func1(0x0, 0x0)
	/tmp/sandbox194689056/main.go:41 +0x40
net/http.(*Request).write.func1(0x10736780, 0x10716f18)
	/usr/local/go/src/net/http/request.go:452 +0x80
net/http.(*Request).write(0x10740880, 0x440890, 0x10733100, 0x0, 0x10733120, 0x0, 0x0, 0x0)
	/usr/local/go/src/net/http/request.go:573 +0x10fb
net/http.(*persistConn).writeLoop(0x10746870, 0x0)
	/usr/local/go/src/net/http/transport.go:1644 +0x200
created by net/http.(*Transport).dialConn
	/usr/local/go/src/net/http/transport.go:1058 +0x13a0