refactor+multi: use *lnwire.FeatureVector for ChannelEdgeInfo features

In this commit, we move the serialisation details of a channel's
features to the DB layer and change the `models` field to instead use a
more useful `*lnwire.FeatureVector` type.

This makes the features easier to work with and moves the serialisation
to where it is actually used.

routing/router_test.go

@@ -2741,6 +2741,7 @@ func TestAddEdgeUnknownVertexes(t *testing.T) {
 		NodeKey2Bytes:    pub2,
 		BitcoinKey1Bytes: pub1,
 		BitcoinKey2Bytes: pub2,
+		Features:         lnwire.EmptyFeatureVector(),
 		AuthProof:        nil,
 	}
 	require.NoError(t, ctx.graph.AddChannelEdge(ctxb, edge))
@@ -2816,6 +2817,7 @@ func TestAddEdgeUnknownVertexes(t *testing.T) {
 
 	edge = &models.ChannelEdgeInfo{
 		ChannelID: chanID.ToUint64(),
+		Features:  lnwire.EmptyFeatureVector(),
 		AuthProof: nil,
 	}
 	copy(edge.NodeKey1Bytes[:], node1Bytes)