channeldb/graphdb: move outpoint ser/deser funcs to graphdb

We have the same helpers for writing and reading a wire.Outpoint type
defined separately in a couple places. We will want to use these from
the graph db package soon though so instead of defining them again
there, this commit unifies things and creates a single exported set of
helpers. The next commit will make use of these.

graph/db/codec.go

@@ -1,8 +1,39 @@
 package graphdb
 
-import "encoding/binary"
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/btcsuite/btcd/wire"
+)
 
 var (
 	// byteOrder defines the preferred byte order, which is Big Endian.
 	byteOrder = binary.BigEndian
 )
+
+// WriteOutpoint writes an outpoint to the passed writer using the minimal
+// amount of bytes possible.
+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 that was previously
+// written using the WriteOutpoint struct.
+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
+}