Refactor JpaPagingItemReader to be compliant with the requirements of constructor

[maequise]

As the new signature of the JpaPagingItemReader is the following:

/**
 * Create a new {@link JpaPagingItemReader} instance.
 * @param entityManagerFactory the JPA entity manager factory.
 * @since 6.0
*/
public JpaPagingItemReader(EntityManagerFactory entityManagerFactory) {
	Assert.notNull(entityManagerFactory, "EntityManagerFactory must not be null.");
	this.entityManagerFactory = entityManagerFactory;
}

The same apply to the JpaItemWriter:

/**
 * Create a new {@link JpaItemWriter} instance.
 * @param entityManagerFactory the entity manager factory to use
 * @since 6.0
*/
public JpaItemWriter(EntityManagerFactory entityManagerFactory) {
	Assert.notNull(entityManagerFactory, "EntityManagerFactory must not be null");
	this.entityManagerFactory = entityManagerFactory;
}

We should make the field EntityManagerFactory entitiyManagerFactory final as we expect to get a non-null factory and remove the setter associated to this field.

Also the reader/writer should not change of entityManagerFactory during the process, and we should define a new one if we need to change the source entityManagerFactory.