Genesis Caching

The /genesis endpoint on validators is used during init and should be unchanging. While there are limits in place, it was brought up in a meeting today that we should cache this to avoid the expensive request to the Thornode from misbehaving clients. The only consideration is that during a fork, if validator gateways are not restarted and new fullnodes come online before the old genesis falls out of cache they will fail to start - as such the cache time is set to 5 minutes, which seems sufficient to avoid load for the Thornode without complications for new nodes initializing after a fork.