Update RelationAwareSet to support lazy loading of internalMap

  * Create DomainBasedMap.Getter interface
  * RelationAwareSet and RelationMulValuesIndexedAwareSet use
    DomainBasedMap.Getter to lazy load their internal collection
  * Make fields in RelationAwareSet 'final' whenever possible
Merge pull request #67 from smmf/feature/support-lazy-loading-relation-aware-set

Update RelationAwareSet to support lazy loading of internalMap

* Create DomainBasedMap.Getter interface
  * RelationAwareSet and RelationMulValuesIndexedAwareSet use
    DomainBasedMap.Getter to lazy load their internal collection
  * Make fields in RelationAwareSet 'final' whenever possible