Change Pub-Sub guarantees to at-most-once

[marcinbudny]

Description

After testing the pub-sub functionality on production it became clear that the aim for at-least-once delivery is not very well aligned with soft real-time, in-memory processing approach promoted by proto.actor. Additionally, ensuring at-least-once delivery has proven to increase complexity of the code and configuration significantly. This PR switches to at-most-once guarantee to align better with promoted use cases for the pub-sub.

Reliable stream processing is still relevant use case, and we may introduce a dedicated functionality that supports it in the future. For now we suggest using external infrastructure like Kafka or RabbitMQ in conjunction with proto.actor to ensure at-least-once processing.

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)