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.

lnrpc/walletrpc/config_active.go

@@ -5,6 +5,7 @@ package walletrpc
 import (
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/macaroons"
 	"github.com/lightningnetwork/lnd/sweep"
 )
@@ -30,7 +31,7 @@ type Config struct {
 
 	// FeeEstimator is an instance of the primary fee estimator instance
 	// the WalletKit will use to respond to fee estimation requests.
-	FeeEstimator lnwallet.FeeEstimator
+	FeeEstimator chainfee.Estimator
 
 	// Wallet is the primary wallet that the WalletKit will use to proxy
 	// any relevant requests to.