Pub sub fix for delivery to stopped subscribers + message renames

[marcinbudny]

Description

• Added failed delivery reporting back to TopicActor
• If the subscriber is no longer reachable (was stopped), then topic will automatically unsubscribe it. This fixed the issue with topic actor being no longer usable, after subscriber is stopped without call to Unsubscribe.
• Changed internal message names to be more readable
• BREAKING: PublisherBatchMessage was renamed to PubSubBatch, which is important for the BatchingProducer error handler configuration.
• PubSubDeliveryActor now uses ReenterAfter to improve throughput when multiple topics are delivering messages.
• Pub sub tests moved out to separate project.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)