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.

sweep/sweeper_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
 
 var (
@@ -99,7 +100,7 @@ func createSweeperTestContext(t *testing.T) *sweeperTestContext {
 
 	backend := newMockBackend(notifier)
 
-	estimator := newMockFeeEstimator(10000, lnwallet.FeePerKwFloor)
+	estimator := newMockFeeEstimator(10000, chainfee.FeePerKwFloor)
 
 	publishChan := make(chan wire.MsgTx, 2)
 	ctx := &sweeperTestContext{
@@ -314,7 +315,7 @@ func assertTxSweepsInputs(t *testing.T, sweepTx *wire.MsgTx,
 // NOTE: This assumes that transactions only have one output, as this is the
 // only type of transaction the UtxoSweeper can create at the moment.
 func assertTxFeeRate(t *testing.T, tx *wire.MsgTx,
-	expectedFeeRate lnwallet.SatPerKWeight, inputs ...input.Input) {
+	expectedFeeRate chainfee.SatPerKWeight, inputs ...input.Input) {
 
 	t.Helper()
 
@@ -994,11 +995,11 @@ func TestDifferentFeePreferences(t *testing.T) {
 	// this to ensure the sweeper can broadcast distinct transactions for
 	// each sweep with a different fee preference.
 	lowFeePref := FeePreference{ConfTarget: 12}
-	lowFeeRate := lnwallet.SatPerKWeight(5000)
+	lowFeeRate := chainfee.SatPerKWeight(5000)
 	ctx.estimator.blocksToFee[lowFeePref.ConfTarget] = lowFeeRate
 
 	highFeePref := FeePreference{ConfTarget: 6}
-	highFeeRate := lnwallet.SatPerKWeight(10000)
+	highFeeRate := chainfee.SatPerKWeight(10000)
 	ctx.estimator.blocksToFee[highFeePref.ConfTarget] = highFeeRate
 
 	input1 := spendableInputs[0]
@@ -1116,7 +1117,7 @@ func TestBumpFeeRBF(t *testing.T) {
 	ctx := createSweeperTestContext(t)
 
 	lowFeePref := FeePreference{ConfTarget: 144}
-	lowFeeRate := lnwallet.FeePerKwFloor
+	lowFeeRate := chainfee.FeePerKwFloor
 	ctx.estimator.blocksToFee[lowFeePref.ConfTarget] = lowFeeRate
 
 	// We'll first try to bump the fee of an output currently unknown to the