refactor: remove Subscriber interface

[ar3s3ru]

The eventstore.Subscriber interface is used to support push semantics of Event Processors: new Events are pushed from the Event Store to the Processor, similar to Pub/Sub approach.

Push semantics is not always supported by all the event streaming technologies out there, and generally speaking event streaming platforms prefer pull semantics instead (currently supported by the eventstore.Streamer interface).

We can remove any trace of this interface from the codebase for v0.1.0, and only rely on pull semantics. This makes things harder for Volatile Subscriptions, but we could add some support for the Stream* methods to fetch events from the latest one available.

[ar3s3ru]

Closing this, the Volatile subscription effort is now tracked in #51