When a node receives a response to a heartbeat, it marks the node as alive at the same time as it updates its view of that node's optimes. This occurs here. We only update the node's optimes though if the optimes have moved forward. If the node hasn't moved forward, that's no reason to not consider it alive. This can lead to spurious elections by unnecessarily throwing away liveness information.

There is a mismatch here between heartbeat request and response processing. In heartbeat requests, we do always update liveness information.