jAuth: auth plugin should periodically check the validity of credentials against the auth backend

[Julien]

In the auth coord plugin, we could have a config option named "checkInterval" which will represent the number of seconds after which the auth credentials stored in session should be checked for validity in the auth backend (db, ldap, class, ...).

Setting this parameter to null will disabled periodic validation.

That way, it will be possible to invalidate auth for already connected users (I'm an admin, and I disable a contributor's account for example, then he should be logged out without having to wait for the session to expire).