Replay channel messages

[oestrich]

Follows up #121

Add in a new message for core that will let you replay a channel from a UID. This will let you catch up if your game was briefly offline.

The server will send:

{ 
  "event": "channels/replay",
  "ref": "28523394-6dcf-4c2a-ad1d-2d0ef8bb823b",
  "payload": {
    "channel": "grapevine",
    "since": "b35198e0-07cb-43f9-b644-7bb6640d2c0f"
  }
}

This could then send broadcast messages for anything missed. Since they have a timestamp this might be ok. Alternatively there can be a separate replay response that includes paginated messages.