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.

channeldb/codec.go

@@ -18,32 +18,6 @@ import (
 	"github.com/lightningnetwork/lnd/tlv"
 )
 
-// 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
-}
-
 // UnknownElementType is an error returned when the codec is unable to encode or
 // decode a particular type.
 type UnknownElementType struct {
@@ -99,7 +73,7 @@ func WriteElement(w io.Writer, element interface{}) error {
 		}
 
 	case wire.OutPoint:
-		return writeOutpoint(w, &e)
+		return graphdb.WriteOutpoint(w, &e)
 
 	case lnwire.ShortChannelID:
 		if err := binary.Write(w, byteOrder, e.ToUint64()); err != nil {
@@ -289,7 +263,7 @@ func ReadElement(r io.Reader, element interface{}) error {
 		}
 
 	case *wire.OutPoint:
-		return readOutpoint(r, e)
+		return graphdb.ReadOutpoint(r, e)
 
 	case *lnwire.ShortChannelID:
 		var a uint64