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>

control/controlclient/netmap.go

@@ -33,6 +33,10 @@ type NetworkMap struct {
 	Hostinfo      tailcfg.Hostinfo
 	PacketFilter  filter.Matches
 
+	// DERPMap is the last DERP server map received. It's reused
+	// between updates and should not be modified.
+	DERPMap *tailcfg.DERPMap
+
 	// ACLs
 
 	User   tailcfg.UserID