MINOR: server/event_hdl: add SERVER_ADMIN event

MINOR: server/event_hdl: add SERVER_ADMIN event

Adding a new SERVER event in the event_hdl API.

SERVER_ADMIN is implemented as an advanced server event.
It is published each time the administrative state changes.
(when s->cur_admin changes)

SERVER_ADMIN data is an event_hdl_cb_data_server_admin struct that
provides additional info related to the admin state change, but can
be casted as a regular event_hdl_cb_data_server struct if additional
info is not needed.