chainfee: create new chainfee package extracting fees from lnwallet

In this commit, we create a new chainfee package, that houses all fee
related functionality used within the codebase. The creation of this new
package furthers our long-term goal of extracting functionality from the
bloated `lnwallet` package into new distinct packages. Additionally,
this new packages resolves a class of import cycle that could arise if a
new package that was imported by something in `lnwallet` wanted to use
the existing fee related functions in the prior `lnwallet` package.

lnwallet/interface.go

@@ -10,6 +10,7 @@ import (
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
 	"github.com/btcsuite/btcwallet/wallet/txauthor"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
 
 // AddressType is an enum-like type which denotes the possible address types
@@ -172,7 +173,7 @@ type WalletController interface {
 	// This method also takes the target fee expressed in sat/kw that should
 	// be used when crafting the transaction.
 	SendOutputs(outputs []*wire.TxOut,
-		feeRate SatPerKWeight) (*wire.MsgTx, error)
+		feeRate chainfee.SatPerKWeight) (*wire.MsgTx, error)
 
 	// CreateSimpleTx creates a Bitcoin transaction paying to the specified
 	// outputs. The transaction is not broadcasted to the network. In the
@@ -184,7 +185,7 @@ type WalletController interface {
 	// NOTE: The dryRun argument can be set true to create a tx that
 	// doesn't alter the database. A tx created with this set to true
 	// SHOULD NOT be broadcasted.
-	CreateSimpleTx(outputs []*wire.TxOut, feeRate SatPerKWeight,
+	CreateSimpleTx(outputs []*wire.TxOut, feeRate chainfee.SatPerKWeight,
 		dryRun bool) (*txauthor.AuthoredTx, error)
 
 	// ListUnspentWitness returns all unspent outputs which are version 0