Batch events and timings

[ianb]

Right now we get events (and with #3727 timings) one-by-one to our /events and /timing endpoints. It would be better to batch these, since some events happen very quickly in succession. Batching can be as simple as:

let pendingEvents = [];
let eventTimer = null;
function sendEvent(event) {
  // do normal stuff to prepare the event
  // set the time of the event
  pendingEvents.push(event);
  if (!eventTimer) {
    eventTimer = setTimeout(() => {
      eventTimer = null;
      flushAllEvents();
      pendingEvents = [];
    }, 1000);
  }
}

We should set queueTime to backdate the events. Note that means we should record how long it was between sendEvent() being called, and flushAllEvents() (per-event).

[jaredhirsch]

Rather than using setTimeout, this seems like a good use case for window.requestIdleCallback.