fix: defer watch until the initial processing of list is complete

[shawkins]

Description

closes: #5953

Provides a callback to ProcessorStore.retainAll so that we can defer starting the watch in some circumstances. Not really a bug, just makes testing / behavior more predictable.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change
• [ ] Chore (non-breaking change which doesn't affect codebase; test, version modification, documentation, etc.)

Checklist

• [x] Code contributed by me aligns with current project license: Apache 2.0
• [x] I Added CHANGELOG entry regarding this change
• [ ] I have implemented unit tests to cover my changes
• [ ] I have added/updated the javadocs and other documentation accordingly
• [ ] No new bugs, code smells, etc. in SonarCloud report
• [ ] I tested my code in Kubernetes
• [ ] I tested my code in OpenShift