Pad packets to reduce metadata leaking

[asn-d6]

We need to add padding to all of our packets to reduce how much metadata is leaking. For example, we are currently leaking the precise payload size of our ROOM_MESSAGE packets.

We should probably always pad our packets up to the maximum length allowed by IRC max message size.

We will probably also need to add a termination token to our transport format (like OTR has "." and ",") to separate payload from padding.

Implementation should be careful here, to not break things or fragmentation. Ideally with unittests as well.