=rem #16428 optimize protobuf serializer for master

#16428 for master
yes and no I guess, it seems `PARSER` is a public field so not "too very internals" to touch. 
The biggest gain is caching those here, we could also cache just the `parseFrom`'s instead perhaps ( impl delegates to PARSER directly btw https://github.com/akka/akka/blob/master/akka-cluster/src/main/java/akka/cluster/protobuf/msg/ClusterMessages.java#L503 )