Support for ranged fetch specifications

[darkv]

This patch adds support for fetch specifications with a range to H2 and PostgreSQL plugins taken from the MySQL plugin.

Example:

EOEditingContext ec = ERXEC.newEditingContext();
ERXFetchSpecification<City> fs = new ERXFetchSpecification<City>(City.ENTITY_NAME);
fs.setSortOrderings(City.SIZE.ascs());
fs.setFetchRange(new NSRange(10, 5));
NSArray<City> cities = ec.objectsWithFetchSpecification(fs);

This will fetch rows 10 through 15 from table City.

If by mistake you have set a range and a limit (fs.setFetchLimit()) the range will win and the limit of your fetch specification will be changed to 0.