closes #126

Motivation

When we add new data topics, we would map the block topic to a response topic on the subscriber. It is possible for multiple blocks to go out in one response, however, so the subscriptions would overwrite each other.

This PR first refactors the subscription system to remove the concept of mapping and replace it with the concept of data topics. It then modifies the subscriber to permit multiple data subscribers per topic and ensure that each of these data subscribers is notified when a topic event is published.

Proposed Changes

1. Eliminate the MappableSubscriber interface because there only needs to be one implementation.
2. Introduce TopicData to slightly distinguish data subscriptions from the original topic subscription. This replaces the mappable topic.
3. Rename notifications.SubscribeOn to notifications.SubscribeWithData for clarity.
4. Rename mappable.go to data_subscriber.go and rename mappableSubscriber to TopicDataSubscriber.
5. Replace Notifee's topic with a data field of type TopicData.
6. Replace mappableSubscriber's idMap with TopicDataSubscriber's data which is now a map of topics to multiple TopicDatas.
7. DataSubscriber's OnNext and and OnClose now call the corresponding methods on the internal subscriber once for each TopicData mapped to the original topic. This fixes block sent listener is not called for every block sent #126.
8. Add a test that a listener registered with RegisterBlockSentListener will get called once for every block actually sent (i.e. that block sent listener is not called for every block sent #126 has been fixed). This test failed before the last two changes.

cc @dirkmc