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/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