lnwallet: prevent static fee estimator fees from being modified

Modifying the static fees is not thread safe. In this commit the fees
are made immutable.

test_utils.go

@@ -202,7 +202,7 @@ func createTestPeer(notifier chainntnfs.ChainNotifier,
 		return nil, nil, nil, nil, err
 	}
 
-	estimator := &lnwallet.StaticFeeEstimator{FeePerKW: 12500}
+	estimator := lnwallet.NewStaticFeeEstimator(12500, 0)
 	feePerKw, err := estimator.EstimateFeePerKW(1)
 	if err != nil {
 		return nil, nil, nil, nil, err