XD-2215: Fixed NPE in ZooKeeperModuleMetadataRepository

Modified `ZooKeeperModuleMetadataRepository` to eliminate NPE.

Introduced Id class to `ModuleMetadata`, which is used to identify instances of `ModuleMetadata` in `ModuleMetadataRepository`.

Deployment unit status in `ModuleMetadata` is now final.

Simplified implementation of deployment unit status retrieval.