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
Add backend-jvstm-common

  * This backend contains code that is common to JVSTM-based backends
  * Some refactorings in CodeGenerator
  * Since a VBox in FF holds the owner and the slotName, remove that duplicate
    information from method's parameters.
  * Add API for generic key-value storage support
  * Add common JVSTMBackend manager
DRAFT: add backend jvstm-infinispan

  * Adjust pom dependencies to consider backend-jvstm-common
  * Update the code generation