Enhances Pubsub with Buffer and Bug fixes

Pubsub has been written from Scratch, now has the following new features
1. Better Concurrency, PubSub now has a dedicated Child Actor for every Channel/Topic
2. Pubsub is now Rev aware, it knows which rev the User subscribed on
3. Pubsub has an in-memory buffer, that records all previous revisions for later subscribers
4. In Memory buffer is pluggable, it can be replaced with other solutions in the future
5. Buffer is optional, Collections' channels are created without Buffer, because it is useless in this case.
Bug Fixes:
1. Pubsub excludes the original publisher from subscribers, so the User won't get an update that he did.
2. Subscribe message contains an ActorRef instead of Option of ActorRef, as it was confusing.