The client currently processes notifications one at a time, and sends individual ACKs for each. Unfortunately, the Simple Push server flushes all pending notifications in response to each ACK, meaning it will redeliver notifications that the client is already processing. This creates a lot of chatter if there are several outstanding notifications (e.g., server sends 6 notifications, client queues them for processing and acknowledges 1, server sends the 5 unacknowledged notifications, client queues the 5 for processing, and so on). Processing and acknowledging notifications in batches will also reduce the number of calls to `ppmm.broadcastAsyncMessage` for the Desktop API.

We can also change the server to batch and flush pending notifications periodically, but :nsm already mentioned batching ACKs as a FIXME: https://github.com/mozilla/gecko-dev/blob/master/dom/push/PushService.jsm#L1083

No longer needed with Web Push-style delivery. Acking messages immediately prevents re-transmission on reconnect.