MINOR: server/event_hdl: add SERVER_STATE event

MINOR: server/event_hdl: add SERVER_STATE event

Adding a new SERVER event in the event_hdl API.

SERVER_STATE is implemented as an advanced server event.
It is published each time the server's effective state changes.
(when s->cur_state changes)

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