Expire using prioqueue

So this is basically where I am with this attempt to maintain a pseudo-index inside the dash, to be able to sweep out elements of the display which aren't updated by the websocket stream anymore, for whatever reason. It seems to work feature-wise, but I wouldn't exclude side-effects (I'm not used to writing javascript), and performance has not been taken in account.

I'll add a more couple comments inline where I have doubts and questions.

In any case, many thanks for the guidance @aphyr ! In retrospect, I'm impressed how small this patchset finally is. The code of the dash is really easy to navigate and pleasant to read, congrats !