Upgrade notes for simpleSAMLphp 1.8

The IdP now sends the NotOnOrAfter attribute in LogoutRequest messages.
We now have full support for selecting the correct AssertionConsumerService endpoint based on parameters in the authentication request. As a side effect of this, an IdP may start sending responses to a new AssertionConsumerService endpoint after upgrade. (This should only happen in the case where it sent the response to the wrong endpoint before.)
The SP no longer incorrectly returns PartialLogout as a status code in a LogoutResponse after the local session has expired.