types/netmap: move some mutations earlier, remove, document some fields

And optimize the Persist setting a bit, allocating later and only mutating
fields when there's been a Node change.

Updates #1909

Change-Id: Iaddfd9e88ef76e1d18e8d0a41926eb44d0955312
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/map_test.go

@@ -446,10 +446,11 @@ func TestNetmapForResponse(t *testing.T) {
 			ComputedNameWithHost: "foo",
 		}
 		ms := newTestMapSession(t)
-
-		nm1 := ms.netmapForResponse(&tailcfg.MapResponse{
+		mapRes := &tailcfg.MapResponse{
 			Node: someNode,
-		})
+		}
+		initDisplayNames(mapRes.Node.View(), mapRes)
+		nm1 := ms.netmapForResponse(mapRes)
 		if nm1.SelfNode == nil {
 			t.Fatal("nil Node in 1st netmap")
 		}