Client-side pings needed for the game API

Currently, a client that doesn't send any events will cause other clients to fall arbitrarily far behind in their committed game state. This is because these other clients have no way of knowing whether this client just has no events to deliver, or whether it has delivered events that have stalled in the network.

To fix this, we want clients to send occasional pings when they haven't delivered an event for a second or two. The handling code for this exists; all that's needed is to modify the driver to actually send the pings.