multi-region, bake in lat/long to fixed-region conn blobs, have clients search nearby regions?

[bradfitz]

To improve robustness of static connection tokens, perhaps they should optionally include the derp region's lat/long too, and then if the DERP map changes over time, the client can try the nearest two (Haversine distance probably good enough) to look for the server's public key again

And/or maybe the server should be able to register at two DERP regions.