Refactor Topic.subscribers & Topic.publishers + extra 

Refactor:
- Topic.subscribers and Topic.publishers used to be sets. It was hard to fetch a connection from the topic list if we had only the subscription_id. The topics didn't persist subscription_id. This poll allows this improvements, making other tasks much easier.

Extra:
- Implemented methods both in TopicManager and Topic which allow a connection - no matter if it is a subscriber or publisher.