Table expiry now raises &on_change handlers

Expiry is remprted as ELEMENT_EXPIRED (which was newly introduced), to
allow distinction from table deletion.

src/types.bif

@@ -229,6 +229,7 @@ enum TableChange %{
 	TABLE_ELEMENT_NEW,
 	TABLE_ELEMENT_CHANGED,
 	TABLE_ELEMENT_REMOVED,
+	TABLE_ELEMENT_EXPIRED,
 %}
 
 module Reporter;