This interceptor is used for the Transaction-scoped persistence context when
there is no transaction. In this case the persistence context is used only
for the method call. Interceptor adds in the thread local a new EntityManager
that is closed at the end of the method.
author: Florent Benoit
Creates a new EntityManager before the method and close it at the end.
Parameters: invocationContext - context with useful attributes on the currentinvocation result of the next invocation (to chain interceptors) throws: Exception - if interceptor fails