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.

NOTE: In the current version, this patch breaks the functionality of
the pre-msgpack protocol, because for both protocols the same
msgcodes are used. To still support 'N' we need to use different
msgcodes depending on if 'N' or 'M' is used.