UNOMI-877: Remove all reference to cellar and hazelcast.

[sergehuber]

UNOMI-877: Replace Karaf Cellar and Hazelcast with PersistenceService for Cluster Synchronization

Overview

This Pull Request extracts and refactors the ClusterService code from the unomi-3-dev branch, focusing on modernizing cluster synchronization in Apache Unomi by:

• Replacing legacy solutions (Karaf Cellar and Hazelcast) with the new PersistenceService.
• Improving maintainability and future-proofing the cluster architecture.

Key Changes

• ClusterService Refactor: Legacy synchronization mechanisms have been removed in favor of a simplified and more robust approach based on PersistenceService.
• Temporary Scheduler Implementation: Due to the absence of an updated Scheduler Service, a classic ScheduledExecutor is used as a temporary solution. This will be replaced in a future PR (UNOMI-878).
• Testing: Unit tests are not included in this PR, as they depend on further upgrades to the Persistence layer and will be provided in a subsequent PR.

Next Steps

• Integrate the new Scheduler Service once available (UNOMI-878)
• Add comprehensive unit tests after upgrading related components

---

• I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004