The Heartbeat message is sent by proposers if they have create_empty_blocks=false or create_empty_blocks_interval > 0, while they are waiting (either for txs or for the interval to expire).

It's only purpose is debugging. The receiver just logs the msg, but doesn't do anything else with it.
The message is signed, but we never check the signature.

Since we're not checking signatures, we probably shouldnt sign heartbeats at all, and remove all the associated code for doing so. We also probably don't want to get spammed with heartbeat messages, so perhaps we can put them behind a debug flag configuration all together?