Currently, if one of the JNs has fallen out of sync with the writer (eg because it went down), it will be marked as such until the next log roll. This causes the writer to no longer send any RPCs to it. This means that the JN's metrics will no longer reflect up-to-date information on how far laggy they are.
This patch will introduce a heartbeat() RPC that has no effect except to update the JN's view of the latest committed txid. When the writer is talking to an out-of-sync logger, it will send these heartbeat messages once a second.
In a future patch we can extend the heartbeat functionality so that NNs periodically check their connections to JNs if no edits arrive, such that a fenced NN won't accidentally continue to serve reads indefinitely.