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.

mock.go

@@ -17,6 +17,7 @@ import (
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
 
 // The block height returned by the mock BlockChainIO's GetBestBlock.
@@ -271,13 +272,13 @@ func (*mockWalletController) IsOurAddress(a btcutil.Address) bool {
 }
 
 func (*mockWalletController) SendOutputs(outputs []*wire.TxOut,
-	_ lnwallet.SatPerKWeight) (*wire.MsgTx, error) {
+	_ chainfee.SatPerKWeight) (*wire.MsgTx, error) {
 
 	return nil, nil
 }
 
 func (*mockWalletController) CreateSimpleTx(outputs []*wire.TxOut,
-	_ lnwallet.SatPerKWeight, _ bool) (*txauthor.AuthoredTx, error) {
+	_ chainfee.SatPerKWeight, _ bool) (*txauthor.AuthoredTx, error) {
 
 	return nil, nil
 }