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.

breacharbiter.go

@@ -21,6 +21,7 @@ import (
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
 
 var (
@@ -78,7 +79,7 @@ type BreachConfig struct {
 	// Estimator is used by the breach arbiter to determine an appropriate
 	// fee level when generating, signing, and broadcasting sweep
 	// transactions.
-	Estimator lnwallet.FeeEstimator
+	Estimator chainfee.Estimator
 
 	// GenSweepScript generates the receiving scripts for swept outputs.
 	GenSweepScript func() ([]byte, error)