Refactor CodeGenerator to facilitate the addition of backend-jvstm-common
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