martian: cancel request context when connection is closed

[mmatczuk]

For Martian TCP listener implementation support request context cancelation when downstream connection dies.

[Choraden]

Determining when a connection dies is not a simple task.

For example http.Server starts background read after it consumed all the request body.
If the read fails the context will be cancelled.

func (cr *connReader) backgroundRead() {
	n, err := cr.conn.rwc.Read(cr.byteBuf[:])
	cr.lock()
	if n == 1 {
		cr.hasByte = true
	}
	if ne, ok := err.(net.Error); ok && cr.aborted && ne.Timeout() {
		// Ignore this error. It's the expected error from
		// another goroutine calling abortPendingRead.
	} else if err != nil {
		cr.handleReadError(err)
	}
	cr.aborted = false
	cr.inRead = false
	cr.unlock()
	cr.cond.Broadcast()
}

// requestBodyRemains reports whether future calls to Read
// on rc might yield more data.
func requestBodyRemains(rc io.ReadCloser) bool {
	if rc == NoBody {
		return false
	}
	switch v := rc.(type) {
	case *expectContinueReader:
		return requestBodyRemains(v.readCloser)
	case *body:
		return v.bodyRemains()
	default:
		panic("unexpected type " + fmt.Sprintf("%T", rc))
	}
}

[Choraden]

The way it could be implemented is very similar to the http.Server approach. We need:

1. http request body wrapper that would notify about the body being consumed in a round tripper
2. conn wrapper that would have the functionality of a backgroundRead()