Consolidate serializers and deserializers.
Change string payload fields to `[]byte`.

Skip device and message IDs; the string form may be useful for
logging, routing, and storing in maps.
`or/cn`: Rearrange field order in `DeliverReply`.
Implement `Device#Reset()`, `Message#Reset()`.
Consolidate connection node implementation.
Remove `cscn` subprotocol.