ipn: don't log redundant peer stats so often

It was especially bad on our GUI platforms with a frontend that polls it.

No need to log it every few seconds if it's unchanged. Make it slightly
less allocate-y while I'm here.

ipn/loglines_test.go

@@ -83,7 +83,9 @@ func TestLocalLogLines(t *testing.T) {
 		}},
 		LocalAddrs: []string{"idk an address"},
 	}
-	lb.parseWgStatus(status)
+	lb.mu.Lock()
+	lb.parseWgStatusLocked(status)
+	lb.mu.Unlock()
 
 	t.Run("after_peers", testWantRemain())
 }