Gateway IPNS plugin improvements

The plugin is currently very naively implemented. Needed improvements:

• verify the hash of downloaded data (relevant: ipfs-only-hash)
• use 2-3 gateways at once for a single resource (gateways are slow, whichever resolves first wins!)
• add a reasonable timeout so that another plugin can pick the ball if it gets dropped by gateway-ipns
  we're implementing global timeouts in the ServiceWorker instead (#65 (closed))