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.

watchtower/wtwire/create_session.go

@@ -3,7 +3,7 @@ package wtwire
 import (
 	"io"
 
-	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 )
 
@@ -34,7 +34,7 @@ type CreateSession struct {
 	// constructing the justice transaction. All sweep transactions created
 	// for this session must use this value during construction, and the
 	// signatures must implicitly commit to the resulting output values.
-	SweepFeeRate lnwallet.SatPerKWeight
+	SweepFeeRate chainfee.SatPerKWeight
 }
 
 // A compile time check to ensure CreateSession implements the wtwire.Message