Fix possible race condition in websocket heartbeat callback

[dordille]

Under load I was seeing cases where heartbeat_timeout was being called when the session had already been set to null.

[brycekahle]

I appreciate the PR. I'd like to find out the source of race condition instead of just treating the symptom. Do you have any more information on when you were seeing the error?

[Slayer95]

I suppose this happens if the session is closed in the 10 second window for receiving client's ping.

I'd expect calling clearTimeout when unregistering the session to fix the issue cleanly.

[JADurham909]

We are seeing this too when running webpack-dev-server. I will try to get more information.

TypeError: Cannot read property 'close' of null     at WebSocketReceiver.heartbeat_timeout (/Users/james/.npm-packages/lib/node_modules/webpack-dev-server/node_modules/sockjs/lib/trans-websocket.js:157:26)     at null._onTimeout (/Users/james/.npm-packages/lib/node_modules/webpack-dev-server/node_modules/sockjs/lib/trans-websocket.js:75:24)     at Timer.listOnTimeout (timers.js:92:15)

[tmds]

ref #185