- New event classes introduced (authorization, table access, command, server
  initialization and shutdown, global variable access, stored program execution).
- Plugin's event handling function can abort further execution of the server enquiry
  that caused generation of the event.
- Plugin allows to subscribe to the specific subclass.
- Sec Context exposed as a Plugin Service.
- Plugin implementation extended (null_plugin) to support event verification

MTR: test_sec_context, audit_plugin_2