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.

lnwallet/interface.go

@@ -302,6 +302,26 @@ type MessageSigner interface {
 	SignMessage(pubKey *btcec.PublicKey, msg []byte) (*btcec.Signature, error)
 }
 
+// FeeEstimator provides the ability to estimate on-chain transaction fees for
+// various combinations of transaction sizes and desired confirmation time
+// (measured by number of blocks).
+type FeeEstimator interface {
+	// EstimateFeePerByte takes in a target for the number of blocks until
+	// an initial confirmation and returns the estimated fee expressed in
+	// satoshis/byte.
+	EstimateFeePerByte(numBlocks uint32) uint64
+
+	// EstimateFeePerWeight takes in a target for the number of blocks until
+	// an initial confirmation and returns the estimated fee expressed in
+	// satoshis/weight.
+	EstimateFeePerWeight(numBlocks uint32) uint64
+
+	// EstimateConfirmation will return the number of blocks expected for a
+	// transaction to be confirmed given a fee rate in satoshis per
+	// byte.
+	EstimateConfirmation(satPerByte int64) uint32
+}
+
 // WalletDriver represents a "driver" for a particular concrete
 // WalletController implementation. A driver is identified by a globally unique
 // string identifier along with a 'New()' method which is responsible for