all: add Node.HomeDERP int, phase out "127.3.3.40:$region" hack [capver 111]

This deprecates the old "DERP string" packing a DERP region ID into an
IP:port of 127.3.3.40:$REGION_ID and just uses an integer, like
PeerChange.DERPRegion does.

We still support servers sending the old form; they're converted to
the new form internally right when they're read off the network.

Updates #14636

Change-Id: I9427ec071f02a2c6d75ccb0fcbf0ecff9f19f26f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlbase/conn_test.go

@@ -280,7 +280,7 @@ func TestConnMemoryOverhead(t *testing.T) {
 	growthTotal := int64(ms.HeapAlloc) - int64(ms0.HeapAlloc)
 	growthEach := float64(growthTotal) / float64(num)
 	t.Logf("Alloced %v bytes, %.2f B/each", growthTotal, growthEach)
-	const max = 2000
+	const max = 2048
 	if growthEach > max {
 		t.Errorf("allocated more than expected; want max %v bytes/each", max)
 	}