Scopes (sharding/subscriptions)

[grrowl]

Add an optional scope key to action.meta, don't gossip those actions to peers unless they've specifically requested a subscription to that scope.

• This can all be done in the redux-scuttlebutt, without scuttlebutt protocol changes. We control the filter function.
• Linked to SOURCE. We should ensure it matches a valid pubKey too.
• When a peer subscribes, they should get all actions on that scope.

[grrowl]

Thinking back, we might need to extend the underlying protocol to ensure when initial subscription occurs, we replay all old actions, or use a timestamp counter per-scope.