Remove `Message` in favour of `Packet`

[decanus]

Problem

We currently have 2 methods of representing a message that gets sent around within UB. One is the Message type and the other is the Packet protocol buffer. The main differences are the fact that the Message type is "more" type safe and has the last sender of a message.

Acceptance Criteria

We remove message from our codebase and pass around (last_sender, packet) as a tuple where necessary. Functions that are responsible for sending a message take the input parameters as UB specific types and later cast them to those required for the protobuf internally.