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.

peer_test.go

@@ -157,7 +157,7 @@ func TestPeerChannelClosureAcceptFeeInitiator(t *testing.T) {
 			dummyDeliveryScript),
 	}
 
-	estimator := lnwallet.StaticFeeEstimator{FeePerKW: 12500}
+	estimator := lnwallet.NewStaticFeeEstimator(12500, 0)
 	feePerKw, err := estimator.EstimateFeePerKW(1)
 	if err != nil {
 		t.Fatalf("unable to query fee estimator: %v", err)
@@ -447,7 +447,7 @@ func TestPeerChannelClosureFeeNegotiationsInitiator(t *testing.T) {
 		msg: respShutdown,
 	}
 
-	estimator := lnwallet.StaticFeeEstimator{FeePerKW: 12500}
+	estimator := lnwallet.NewStaticFeeEstimator(12500, 0)
 	initiatorIdealFeeRate, err := estimator.EstimateFeePerKW(1)
 	if err != nil {
 		t.Fatalf("unable to query fee estimator: %v", err)