lnwire21: update Msat and TrueBoolean

Add the TrueBoolean type along with its Record method. Also update the
Millisatoshi type with a Record method. Both of these will be used in an
upcoming commit which adjusts a mission control migration to use pure
TLV types.

channeldb/migration/lnwire21/true_boolean.go

@@ -0,0 +1,37 @@
+package lnwire
+
+import (
+	"io"
+
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+// TrueBoolean is a record that indicates true or false using the presence of
+// the record. If the record is absent, it indicates false. If it is present,
+// it indicates true.
+type TrueBoolean struct{}
+
+// Record returns the tlv record for the boolean entry.
+func (b *TrueBoolean) Record() tlv.Record {
+	return tlv.MakeStaticRecord(
+		0, b, 0, booleanEncoder, booleanDecoder,
+	)
+}
+
+func booleanEncoder(_ io.Writer, val interface{}, _ *[8]byte) error {
+	if _, ok := val.(*TrueBoolean); ok {
+		return nil
+	}
+
+	return tlv.NewTypeForEncodingErr(val, "TrueBoolean")
+}
+
+func booleanDecoder(_ io.Reader, val interface{}, _ *[8]byte,
+	l uint64) error {
+
+	if _, ok := val.(*TrueBoolean); ok && (l == 0 || l == 1) {
+		return nil
+	}
+
+	return tlv.NewTypeForEncodingErr(val, "TrueBoolean")
+}