RAW/SDS: Reconsider message and channel id fields for Message type

[adklempner]

Per @jm-clius (#130 (comment)) :

While we're here, should we review the field types for the rest of the Message? The choice of string for message_id and channel_id was heavily influenced by similar protocols and current Status code, but might not be the most practical from implementation perspective: I can imagine bytes perhaps being more reasonable for message_id and int32 for channel_id.

Any opinions here? cc @shash256