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.

contractcourt/chain_arbitrator.go

@@ -13,6 +13,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/sweep"
 )
@@ -131,7 +132,7 @@ type ChainArbitratorConfig struct {
 	Signer input.Signer
 
 	// FeeEstimator will be used to return fee estimates.
-	FeeEstimator lnwallet.FeeEstimator
+	FeeEstimator chainfee.Estimator
 
 	// ChainIO allows us to query the state of the current main chain.
 	ChainIO lnwallet.BlockChainIO