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.

Utility for reporting client requests from ZK log