eap-radius: Add cache for usage stats of expired/rekeyed SAs

eap-radius: Add cache for usage stats of expired/rekeyed SAs

There are several situations that the previous code didn't handle that
well, for example, interim updates during rekeying (until the rekeyed SA
was deleted the numbers were too high, then suddenly dropped afterwards),
or rekeying for IKEv1 in general because rekeyed IPsec SAs stay installed
until they expire (so if they were still around when the IKE_SA was
terminated, the reported numbers in the Stop message were too high).

If intermediate updates are not used the cache entries for rekeyed
CHILD_SA will accumulate, we can't clean them up as we don't get
child_updown() events for them.