ProtobufRepresentable for handshakes and use it in TransportPipelines

[ktoso]

the handshakes predate a lot of other things.

• They should use ProtobufRepresentable (extend it)
• the system.serialization should be used in TransportPipelines to serialize them, rather the raw serialization:

            let bytes: ByteBuffer = try proto.serializedByteBuffer(allocator: context.channel.allocator)
             // TODO: should we use the serialization infra ourselves here? I guess so...