Be more robust when an exception is thrown in TransactionService.begin()
Inlined TransactionContent for DeleteUserMemberShip
TransactionService refactoring

Cleaned up the interface :
- deprecated the method isTransactionActive
- deleted the method getBonitaSynchronisations (never used)

Added the new method : executeInTransaction(Callable) in order to replace the
TransactionExecutor class.
PlatformAPIImpl do not use TransactionExecutor anymore.
No HibernateException should go outside the Bonita API.
Added comments and stopped using the TransactionExecutor.
Added comment : the method closeTransactionIfOpen should not survive the transaction refactoring.
Do not check if there is an active transaction or not : this is the job of
the caller to ensure that a transaction is open.
Use the TransactionService instead of the TransactionExecutor (it will disappear).