graph/db: split HasNode into two methods

HasNode currently is very v1 specific since it returns a time.Time
timestamp which is specific to V1 node announcements. However, it is
mostly only used for the "exists" return value. So here we split it up
into HasNode which just checks existence and HasV1Node which retains the
same behavaiour as before.

graph/db/graph.go

@@ -664,10 +664,19 @@ func (c *ChannelGraph) FetchNode(ctx context.Context,
 	return c.db.FetchNode(ctx, nodePub)
 }
 
-// HasNode determines if the graph has a vertex identified by the target node.
-func (c *ChannelGraph) HasNode(ctx context.Context,
+// HasV1Node determines if the graph has a vertex identified by the target node
+// in the V1 graph.
+func (c *ChannelGraph) HasV1Node(ctx context.Context,
 	nodePub [33]byte) (time.Time, bool, error) {
 
+	return c.db.HasV1Node(ctx, nodePub)
+}
+
+// HasNode determines if the graph has a vertex identified by the target node
+// in the V1 graph.
+func (c *ChannelGraph) HasNode(ctx context.Context, nodePub [33]byte) (bool,
+	error) {
+
 	return c.db.HasNode(ctx, nodePub)
 }