Minor changes in core (SharedIdentityMap)

  * Enable the SharedIdentityMap to print its contents
Fix: Throw RollbackException when Transaction.commit fails

  * The failed commit was being caught and silenced, which could not be
    detected by whoever called commit.  Now, it throws a RollbackException.
    We might even consider letting the TransactionError pass up if needed.
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
DRAFT: backend-jvstm-common remove commented code in jvstmcodegenerator
DRAFT: backend-jvstm-mem remove deleted code that no longer overrides
DRAFT: make backend-jvstm-ispn extend from jvstm-common
DRAFT: backend-jvstm-common tx+oids+domainclassinfo
DRAFT: backend-jvstm-common replace Error by RuntimeException in withTxNoException
Add tests-backend-jvstm-common