ADFS iframe logout handling broken #2555

During logout via the iframe logout handler the ADFS associations are not terminated.

Module adfs prefixes its entityIDs with adfs:, but the LogoutController only looks for keys prefixed with saml:.

Possible fixes:

1. dynamically resolve entityID using the same mechanism to determine the metadata set
2. change the entityID prefix in module adfs to 'saml'
3. use the hook infrastructure to define a protocol agnostic interface for logout scenarios

Any ideas or preferences on how to solve this?