See discussion at #1067.

Allow registration of own (de)serializer for extension objects that are not in the ua namespace.
(see original TODO in code)

example:
```python
@dataclass
class YourDataType:

    data_type = ua.NodeId.from_string('''ns=2;i=3007''')

    Encoding: ua.UInt32 = field(default=0, repr=False, init=False, compare=False)
    ID: ua.String = ua.String()
    Description: Optional[ua.String] = None

ua.register_extension_object("YourDataType",
                                               ua.NodeId.from_string( f"ns=2;i=5002"),
                                               YourDataType,
                                                YourDataType.data_type)
```

When event is save it is added to  self._events[event.emitting_node], only that list is never initialized.

Makes typecheckers recognise the subclasses as Exceptions (#1204).

make sure we disconnect (in reality kill ThreadLoop) when we get an exception in `__enter__`in sync wrapper

Exceptions have already been handled and logged in _monitor_server_loop / _renew_channel_loop,
there is no need to log it again with higher loglevel

[xmlexporter] fix extensionobjects typeid indentifier missing namespace mapping to idx_in_exported_file

Other nodeids namespace are corrected to ``dx_in_exported_file`, only  the extensionobject typeid identifier isn't. Resulting in bad export.

Example:
```
<uax:ListOfExtensionObject>
        <uax:ExtensionObject>
          <uax:TypeId>
            <uax:Identifier>i=296</uax:Identifier>
          </uax:TypeId>
          <uax:Body>
            <uax:Argument>
              <uax:Name>MyFiled</uax:Name>
              <uax:DataType>
                <uax:Identifier>ns=3;i=3003</uax:Identifier>
              </uax:DataType>
```
In the case above `<uax:Identifier>ns=3;i=3003</uax:Identifier>` isn't corrected.
This can be easly fixed in `XmlExporter._val_to_etree` by using the lookup table `self._addr_idx_to_xml_idx`.

* Update server.py

* Update server.py

* add: unregister

* update: register procedure

* update: uaprocessor

* add: fixme

* fix: test

* fix: tests

Fixes #1180

OPC-UA specification Part 4, 5.6.3 specifies that a Null or empty user
token shall always be interpreted as anonymous. Add a test for this case
and a fix to properly handle it.

do not print a very long exception trace when a handle does not have status_change_notification method

Certificates, that are already loaded into memory as bytes, can now directly be handed over
to set_security and load_certificate.