lnwallet: add FeeEstimator interface, StaticFeeEstimator implementation

This commit adds the FeeEstimator interface, which can be used for
future fee calculation implementations. Currently, there is only the
StaticFeeEstimator implementation, which returns the same fee rate for
any transaction.

lnd.go

@@ -149,11 +149,12 @@ func lndMain() error {
 	signer := wc
 	bio := wc
 	fundingSigner := wc
+	estimator := lnwallet.StaticFeeEstimator{FeeRate: 250}
 
 	// Create, and start the lnwallet, which handles the core payment
 	// channel logic, and exposes control via proxy state machines.
 	wallet, err := lnwallet.NewLightningWallet(chanDB, notifier, wc, signer,
-		bio, activeNetParams.Params)
+		bio, estimator, activeNetParams.Params)
 	if err != nil {
 		fmt.Printf("unable to create wallet: %v\n", err)
 		return err
@@ -191,7 +192,7 @@ func lndMain() error {
 	// With all the relevant chains initialized, we can finally start the
 	// server itself.
 	server, err := newServer(defaultListenAddrs, notifier, bio,
-		fundingSigner, wallet, chanDB, chainView)
+		fundingSigner, wallet, estimator, chanDB, chainView)
 	if err != nil {
 		srvrLog.Errorf("unable to create server: %v\n", err)
 		return err