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.

lnwallet/log.go

@@ -7,6 +7,7 @@ import (
 	"github.com/btcsuite/btcwallet/wtxmgr"
 
 	"github.com/lightningnetwork/lnd/build"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
 
 // walletLog is a logger that is initialized with no output filters.  This
@@ -34,6 +35,7 @@ func UseLogger(logger btclog.Logger) {
 	btcwallet.UseLogger(logger)
 	wtxmgr.UseLogger(logger)
 	chain.UseLogger(logger)
+	chainfee.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations