Akka.Cluster: need to enable HeartbeatResponse message type

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

First introduced in Akka.NET v1.4.19 via #4934, we need to enable the new HeartbeatRsp message to be sent by default. All clients going back to v1.4.19 and earlier have the ability to deserialize this message, but we never introduced the response payload since it could result in a break in wire compatibility.

Thus, we followed our best practice and allowed the read-side code to "age into" existing installations over a long-ish period of time. Now with Akka.NET v1.5 come up it's the right time to enable this particular feature as it will ultimately assist with debugging heartbeat issues inside Akka.Cluster.

Describe the solution you'd like

Need to update the ClusterMessageSerializer to produce the typed protobuf HeartbeatResponse message instead of the old UniqueAddress response message we currently write here:

akka.net/src/core/Akka.Cluster/Serialization/ClusterMessageSerializer.cs

Line 72 in 03de287

return UniqueAddressToProto(heartbeatRsp.From).ToByteArray();

You can see what the typed HeartbeatResponse message looks like here:

akka.net/src/protobuf/ClusterMessages.proto

Lines 73 to 83 in 03de287

	/**
	* Prior to version 1.4.19
	* HeartbeatRsp
	* Sends an UniqueAddress
	* Version 1.4.19 can deserialize this message but does not send it
	*/
	message HeartBeatResponse {
	UniqueAddress from = 1;
	int64 sequenceNr = 2;
	int64 creationTime = 3;
	}

Describe alternatives you've considered

Leaving this as-is and making it opt-in, but we decided against that in Akka.NET v1.4.19. It should be safe for most clusters to just upgrade straight v1.5.0.