fix(server): reduce the screaming heartbeat logs

Currently the only way we know that a peer isn't getting a heartbeat is an
edge triggered event from go raft on every missed heartbeat. This means that
we need to do some book keeping in order to do exponential backoff.

The upside is that instead of screaming thousands of log lines before a 
machine hits the default removal of 30 minutes it is only ~100.
If the peer is not `*raft.Peer`, it will record it as `<unknown>` in the log and backoff map. But we never delete it in the `monitorLogBackoff`.