Session Expiry in CONNACK should be optional

The Session Expiry returned value in the CONNACK is currently not optional. I think it ought to be optional, its presence indicated by a flag. It takes 4 bytes, and in many (most?) the Gateway will not change the value sent by the client on the CONNECT.

Also, the current approach uses a special value, 0, to indicate that the value in the CONNECT packet is being used. If Session Expiry is optional, this condition is changed to "is present", a much cleaner definition.