The OPC UA specification does not forbid that a server does not offer DiscoveryUrls. This can lead to a crash when calling the function application_to_strings().

Check DiscoveryUrls for None or empty before iterating to avoid the crash described in #1127.

* extended ServerItemCallback class with 'event_comes_from_internal_session' parameter to distinguish events triggered from server to the ones triggered from OPC-UA clients

* Fixed arguments order

* Renamed variable to is_external
Co-authored-by: Marco Panato <marco.panato_01@univr.it>

* Remove old uniqueness check
Add check that properties are unique in one hierarchy

* Replace old duplicates browsename test with duplicated property browsename test.
Remove duplicated browsename test with different ns.

Co-authored-by: jportela <joao.portela@asml.com>

- Make ReturnBounds parameter configurable
- Make possible to read history recursively

Co-authored-by: oroulet <olivier@r-dt.net>