Improve Pub/Sub MQTT driver context propagation

[adriansmares]

Summary

Closes #3338

Changes

• Upgrade Paho to ba85050a1f23
• Remove connection / publishing / subscription timeouts. Use context cancellations instead, as Paho recently added support for it (only await, no cancellation for started operations unfortunately).
• Always disconnect the client if the connection attempt fails
• Linking and Pub/Sub tasks no longer stop on registry errors (except NotFound errors) - we've actually observed this in production when the link to Redis stops, and the task stops
• Do not cancel contexts with nil error. This makes code that uses ctx.Done and ctx.Err to miss the cancellation
• Add keep-alive to MQTT connections

Testing

Unit testing covers this.

Regressions

As this touches the MQTT driver timeouts, and a bit of the linking/pub/sub startup logic, regressions can occur there. However, the changes actually make us more lenient in task restarts (since we now actually propagate the errors better).

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[htdvisser]

Probably smart to keep a replacement to the latest master, otherwise a dependency upgrade may accidentally revert it to v1.2.0.