OIDC: implement Back-Channel and Front-Channel logout

Implement new standards in OpenID Connect to manage logout :

• http://openid.net/specs/openid-connect-frontchannel-1_0.html
• http://openid.net/specs/openid-connect-backchannel-1_0.html

Both are needed: front-channel to avoid local cache timeout, back-channel for RP1->OP->RP2 logout