all: make client use server-provided DERP map, add DERP region support

Instead of hard-coding the DERP map (except for cmd/tailscale netcheck
for now), get it from the control server at runtime.

And make the DERP map support multiple nodes per region with clients
picking the first one that's available. (The server will balance the
order presented to clients for load balancing)

This deletes the stunner package, merging it into the netcheck package
instead, to minimize all the config hooks that would've been
required.

Also fix some test flakes & races.

Fixes #387 (Don't hard-code the DERP map)
Updates #388 (Add DERP region support)
Fixes #399 (wgengine: flaky tests)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -315,8 +315,9 @@ type NetInfo struct {
 	LinkType string // "wired", "wifi", "mobile" (LTE, 4G, 3G, etc)
 
 	// DERPLatency is the fastest recent time to reach various
-	// DERP STUN servers, in seconds. The map key is the DERP
-	// server's STUN host:port.
+	// DERP STUN servers, in seconds. The map key is the
+	// "regionID-v4" or "-v6"; it was previously the DERP server's
+	// STUN host:port.
 	//
 	// This should only be updated rarely, or when there's a
 	// material change, as any change here also gets uploaded to
@@ -336,7 +337,7 @@ func (ni *NetInfo) String() string {
 }
 
 // BasicallyEqual reports whether ni and ni2 are basically equal, ignoring
-// changes in DERPLatency.
+// changes in DERP ServerLatency & RegionLatency.
 func (ni *NetInfo) BasicallyEqual(ni2 *NetInfo) bool {
 	if (ni == nil) != (ni2 == nil) {
 		return false