Move/refactor keepalive logic and add disconnect timer

We now disconnect ourselves if we don't get the server's response to a
keepalive request within 30s. This way we will eventually disconnect if
the network goes away but the socket is not closed.*

* See code.google.com/p/chromium/issues/detail?id=197841 and
https://stackoverflow.com/questions/11755605/chrome-websocket-connection-not-closed-when-browser-closed

We will then try to reconnect once a minute (See 8a10c96);

Keepalives belong at this level anyway, since the format is defined by
both the websocket resource protocol and our specific server url
structure.

// FREEBIE

libtextsecure/message_receiver.js

@@ -44,9 +44,26 @@
                 }
             }
 
-            new WebSocketResource(this.socket, this.handleRequest.bind(this));
+            this.wsr = new WebSocketResource(this.socket, this.handleRequest.bind(this));
+            this.resetKeepAliveTimer();
+
+        },
+        resetKeepAliveTimer: function() {
+            clearTimeout(this.keepAliveTimer);
+            clearTimeout(this.disconnectTimer);
+            this.keepAliveTimer = setTimeout(function() {
+                if (this.getStatus() === WebSocket.OPEN) {
+                    this.wsr.sendRequest({
+                        verb: 'GET',
+                        path: '/v1/keepalive',
+                        success: this.resetKeepAliveTimer.bind(this)
+                    });
+                }
+                this.disconnectTimer = setTimeout(this.socket.close, 30000);
+            }.bind(this), 55000);
         },
         handleRequest: function(request) {
+            this.resetKeepAliveTimer();
             // TODO: handle different types of requests. for now we only expect
             // PUT /messages <encrypted IncomingPushMessageSignal>
             textsecure.crypto.decryptWebsocketMessage(request.body).then(function(plaintext) {