Update: Add the basic structure for an order header.

Update: Add the basic structure for an order header.

An order header helps achieve something like serialization.
This is different from the packet number.
The order represets the order in regards to the packets sent and not the specific data (such as `part`).
For example, if a retry is requested for the same packet, then the order will have increased but the actual `part` number would be the same.