Root key refresh

[FR4NK-W]

A RAINS server needs the public key of the root rains server to bootstrap trust. Currently the root public key is loaded when the server starts up and the delegation assertion containing the root public key will expire after some time. At this point, it is not possible for the RAINS server to validate any signature as the root public key is missing. Make sure that the RAINS server queries the root for an updated delegation assertion before the current one expires.