channeldb: MigrateOutpointIndex, store indexStatus in outpoint index

Adds an outpoint index that stores a tlv stream. Currently the stream
only contains the outpoint's indexStatus. This should cut down on
big bbolt transactions in several places throughout the codebase.

channeldb/migration20/codec.go

@@ -0,0 +1,36 @@
+package migration20
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/btcsuite/btcd/wire"
+)
+
+var (
+	byteOrder = binary.BigEndian
+)
+
+// writeOutpoint writes an outpoint from the passed writer.
+func writeOutpoint(w io.Writer, o *wire.OutPoint) error {
+	if _, err := w.Write(o.Hash[:]); err != nil {
+		return err
+	}
+	if err := binary.Write(w, byteOrder, o.Index); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// readOutpoint reads an outpoint from the passed reader.
+func readOutpoint(r io.Reader, o *wire.OutPoint) error {
+	if _, err := io.ReadFull(r, o.Hash[:]); err != nil {
+		return err
+	}
+	if err := binary.Read(r, byteOrder, &o.Index); err != nil {
+		return err
+	}
+
+	return nil
+}