go/neo/proto: Update msgcode increment logic to new protocol

In pre-msgpack protocol, the msgcode increment logic is like this:

Notify 	 add +1 to next msgcode
Request  add +0 to next msgcode   (next msg is answer & should therefore be the same)
Answer 	 add +1 to next msgcode

('Answer' msgcode is adjusted by 'AnswerBit')

In post-msgpack protocol, the logic is a bit different:

Notify 	 add +1 to next msgcode
Request  add +0 to next msgcode   (next msg is answer & should therefore be the same)
Answer 	 add +2 to next msgcode

So here we produce gaps after Request/Answer pairs.

go/neo/proto/protogen.go

@@ -371,12 +371,18 @@ import (
 			// generate code for this type to implement neo.Msg
 			var msgCode MsgCode
 			msgCode.answer = specAnnotation.answer || strings.HasPrefix(typename, "Answer")
+
+			msgCode.msgSerial = msgSerial
+
 			// increment msgSerial only by +1 when going from
 			// request1->request2 in `Request1 Answer1 Request2`.
+			// Unlike as it was in pre-msgpack protocol, the global
+			// increment is still +1, only for the answer packet
+			// itself it's the same as the request packet. This means
+			// in the post-msgpack protocol there are gaps.
 			if msgCode.answer && typename != "Error" {
-				msgSerial--
+				msgCode.msgSerial = msgSerial - 1
 			}
-			msgCode.msgSerial = msgSerial
 
 			fmt.Fprintf(&buf, "// %s. %s\n\n", msgCode, typename)