HA plugin should sync last sent packet
If a failover occurs between sending a message and receiving the response, then it is possible that the peer has actually received the message.
Then after the failover, the new instance sends a different message with the same counter, and the peer will ignore it forever.
Similarly, on the responder:
If a failover occurs after sending an ack, but the peer did not receive the ack, the peer will resend its original message and the new instance should send the exact same answer as before (but it won't have it).