go.tools/playground/socket: log connections and code snippet run requests

This logs all successful handshakes and all requests to run code snippets;
it is not immediately obvious how to limit this to non-localhost hosts, or
to instances where publicly available playgrounds are allowed without resorting
to addition of a new global. The level of noise on the log should not be too
great.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/112850043

playground/socket/socket.go

@@ -95,6 +95,7 @@ func handshake(c *websocket.Config, req *http.Request) error {
 		log.Println("bad websocket origin:", o)
 		return websocket.ErrBadWebSocketOrigin
 	}
+	log.Println("accepting connection from:", req.RemoteAddr)
 	return nil
 }
 
@@ -136,6 +137,7 @@ func socketHandler(c *websocket.Conn) {
 		case m := <-in:
 			switch m.Kind {
 			case "run":
+				log.Println("running snippet from:", c.Request().RemoteAddr)
 				proc[m.Id].Kill()
 				lOut := limiter(in, out)
 				proc[m.Id] = startProcess(m.Id, m.Body, lOut, m.Options)