RPKI: refactore and improve

Takes into account a preferences of caches.
Fixes many bugs.

Configuration example:

    roa table roatable;

    protocol rpki {
      debug all;
      roa table roatable;

      cache "rpki-validator.realmv6.org" {
          port 3233;
          preference 1;
      };
      cache 127.0.0.1 {
          preference 2;
      };
      cache "rpki-validator.realmv6.org";
    }