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/rates.go

@@ -0,0 +1,53 @@
+package chainfee
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/blockchain"
+	"github.com/btcsuite/btcutil"
+)
+
+const (
+	// FeePerKwFloor is the lowest fee rate in sat/kw that we should use for
+	// determining transaction fees.
+	FeePerKwFloor SatPerKWeight = 253
+)
+
+// SatPerKVByte represents a fee rate in sat/kb.
+type SatPerKVByte btcutil.Amount
+
+// FeeForVSize calculates the fee resulting from this fee rate and the given
+// vsize in vbytes.
+func (s SatPerKVByte) FeeForVSize(vbytes int64) btcutil.Amount {
+	return btcutil.Amount(s) * btcutil.Amount(vbytes) / 1000
+}
+
+// FeePerKWeight converts the current fee rate from sat/kb to sat/kw.
+func (s SatPerKVByte) FeePerKWeight() SatPerKWeight {
+	return SatPerKWeight(s / blockchain.WitnessScaleFactor)
+}
+
+// String returns a human-readable string of the fee rate.
+func (s SatPerKVByte) String() string {
+	return fmt.Sprintf("%v sat/kb", int64(s))
+}
+
+// SatPerKWeight represents a fee rate in sat/kw.
+type SatPerKWeight btcutil.Amount
+
+// FeeForWeight calculates the fee resulting from this fee rate and the given
+// weight in weight units (wu).
+func (s SatPerKWeight) FeeForWeight(wu int64) btcutil.Amount {
+	// The resulting fee is rounded down, as specified in BOLT#03.
+	return btcutil.Amount(s) * btcutil.Amount(wu) / 1000
+}
+
+// FeePerKVByte converts the current fee rate from sat/kw to sat/kb.
+func (s SatPerKWeight) FeePerKVByte() SatPerKVByte {
+	return SatPerKVByte(s * blockchain.WitnessScaleFactor)
+}
+
+// String returns a human-readable string of the fee rate.
+func (s SatPerKWeight) String() string {
+	return fmt.Sprintf("%v sat/kw", int64(s))
+}