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/reports_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
+	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/stretchr/testify/require"
 )
@@ -137,7 +138,7 @@ func TestFetchChannelWriteBucket(t *testing.T) {
 		error) {
 
 		var chanPointBuf bytes.Buffer
-		err := writeOutpoint(&chanPointBuf, &testChanPoint1)
+		err := graphdb.WriteOutpoint(&chanPointBuf, &testChanPoint1)
 		require.NoError(t, err)
 
 		return chainHash.CreateBucketIfNotExists(chanPointBuf.Bytes())