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/chainfee/log.go

@@ -0,0 +1,29 @@
+package chainfee
+
+import (
+	"github.com/btcsuite/btclog"
+	"github.com/lightningnetwork/lnd/build"
+)
+
+// log is a logger that is initialized with no output filters.  This means the
+// package will not perform any logging by default until the caller requests
+// it.
+var log btclog.Logger
+
+// The default amount of logging is none.
+func init() {
+	UseLogger(build.NewSubLogger("CFEE", nil))
+}
+
+// DisableLog disables all library log output.  Logging output is disabled by
+// default until UseLogger is called.
+func DisableLog() {
+	UseLogger(btclog.Disabled)
+}
+
+// UseLogger uses a specified Logger to output package logging info.  This
+// should be used in preference to SetLogWriter if the caller is also using
+// btclog.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}