xP: use the binary protocol for incoming events

And batch event messages together as much as possible.

JSON has proven itself to be really slow
(for example, encoding/json.Marshaler is a slow interface),
and browsers have significant overhead per WS message.

Commands are still sent as JSON, sending them in binary
would be a laborious rewrite without measurable merits.

The xP server now only prints debug output when requested,
because that was another source of major slowdowns.

xP/.gitignore

@@ -1,3 +1,4 @@
 /xP
 /proto.go
+/public/proto.js
 /public/mithril.js