Draft "channel keepalive" protocol to identify dead nodes and rotate session keys with inactive nodes

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Executive summary

We need a way know when a peer is dead so we can free up resources (e.g., session keys) allocated to that peer. This requires us to tell apart nodes that are dead from nodes that are still alive but simply there's not been any need to exchange data recently. A simple keepalive protocol (like TCP's) would satisfy this, and it'd also allow us to continue rotating session keys.