- `EventSubscribers` ADT is now an immutable (case class)
- Refactored `SubscribersKeeperActor` a bit for readability.
- Changed message sent by `SubscribersKeeperActor` in response to `GetSubscribers` from a bare `List[String]` to an `EventSubscribers`.
- Modified response JSON format for `GET /v2/event_subscriptions` for consistency.
- Modified key in serialized form of SubscriptionEvents for uniformity (URL vs URI).