ikev2: Add a new state to track rekeyed IKE_SAs

This makes handling such IKE_SAs more specifically compared to keeping them
in state IKE_CONNECTING or IKE_ESTABLISHED (which we did when we lost a
collision - even triggering the ike_updown event), or using IKE_REKEYING for
them, which would also be ambiguous.

For instance, we can now reject anything but DELETES for such SAs.

src/libcharon/sa/ike_sa.h

@@ -308,6 +308,11 @@ enum ike_sa_state_t {
 	 */
 	IKE_REKEYING,
 
+	/**
+	 * IKE_SA has been rekeyed (or is redundant)
+	 */
+	IKE_REKEYED,
+
 	/**
 	 * IKE_SA is in progress of deletion
 	 */