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.

watchtower/wtclient/backup_task_internal_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
@@ -86,7 +87,7 @@ func genTaskTest(
 	toLocalAmt int64,
 	toRemoteAmt int64,
 	blobType blob.Type,
-	sweepFeeRate lnwallet.SatPerKWeight,
+	sweepFeeRate chainfee.SatPerKWeight,
 	rewardScript []byte,
 	expSweepAmt int64,
 	expRewardAmt int64,

watchtower/wtdb/codec.go

@@ -4,7 +4,7 @@ import (
 	"io"
 
 	"github.com/lightningnetwork/lnd/channeldb"
-	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 	"github.com/lightningnetwork/lnd/watchtower/wtpolicy"
 )
@@ -58,7 +58,7 @@ func ReadElement(r io.Reader, element interface{}) error {
 		}
 
 		e.BlobType = blob.Type(blobType)
-		e.SweepFeeRate = lnwallet.SatPerKWeight(sweepFeeRate)
+		e.SweepFeeRate = chainfee.SatPerKWeight(sweepFeeRate)
 
 	// Type is still unknown to wtdb extensions, fail.
 	default:

watchtower/wtpolicy/policy.go

@@ -7,6 +7,7 @@ import (
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 )
 
@@ -27,11 +28,11 @@ const (
 
 	// DefaultSweepFeeRate specifies the fee rate used to construct justice
 	// transactions. The value is expressed in satoshis per kilo-weight.
-	DefaultSweepFeeRate = lnwallet.SatPerKWeight(2500)
+	DefaultSweepFeeRate = chainfee.SatPerKWeight(2500)
 
 	// MinSweepFeeRate is the minimum sweep fee rate a client may use in its
 	// policy, the current value is 4 sat/vbyte.
-	MinSweepFeeRate = lnwallet.SatPerKWeight(1000)
+	MinSweepFeeRate = chainfee.SatPerKWeight(1000)
 )
 
 var (
@@ -97,7 +98,7 @@ type TxPolicy struct {
 	// constructing the justice transaction. All sweep transactions created
 	// for this session must use this value during construction, and the
 	// signatures must implicitly commit to the resulting output values.
-	SweepFeeRate lnwallet.SatPerKWeight
+	SweepFeeRate chainfee.SatPerKWeight
 }
 
 // Policy defines the negotiated parameters for a session between a client and

watchtower/wtwire/create_session.go

@@ -3,7 +3,7 @@ package wtwire
 import (
 	"io"
 
-	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 )
 
@@ -34,7 +34,7 @@ type CreateSession struct {
 	// constructing the justice transaction. All sweep transactions created
 	// for this session must use this value during construction, and the
 	// signatures must implicitly commit to the resulting output values.
-	SweepFeeRate lnwallet.SatPerKWeight
+	SweepFeeRate chainfee.SatPerKWeight
 }
 
 // A compile time check to ensure CreateSession implements the wtwire.Message

watchtower/wtwire/wtwire.go

@@ -8,7 +8,7 @@ import (
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 )
@@ -74,7 +74,7 @@ func WriteElement(w io.Writer, element interface{}) error {
 			return err
 		}
 
-	case lnwallet.SatPerKWeight:
+	case chainfee.SatPerKWeight:
 		var b [8]byte
 		binary.BigEndian.PutUint64(b[:], uint64(e))
 		if _, err := w.Write(b[:]); err != nil {
@@ -194,12 +194,12 @@ func ReadElement(r io.Reader, element interface{}) error {
 		}
 		*e = bytes
 
-	case *lnwallet.SatPerKWeight:
+	case *chainfee.SatPerKWeight:
 		var b [8]byte
 		if _, err := io.ReadFull(r, b[:]); err != nil {
 			return err
 		}
-		*e = lnwallet.SatPerKWeight(binary.BigEndian.Uint64(b[:]))
+		*e = chainfee.SatPerKWeight(binary.BigEndian.Uint64(b[:]))
 
 	case *ErrorCode:
 		var b [2]byte