graph: add IsZombieEdge method

This is in preparation for the commit where we move across all the
funding tx validation so that we can test that we are correctly updating
the zombie index.

graph/interfaces.go

@@ -88,6 +88,10 @@ type ChannelGraphSource interface {
 
 	// MarkZombieEdge marks the channel with the given ID as a zombie edge.
 	MarkZombieEdge(chanID uint64) error
+
+	// IsZombieEdge returns true if the edge with the given channel ID is
+	// currently marked as a zombie edge.
+	IsZombieEdge(chanID lnwire.ShortChannelID) (bool, error)
 }
 
 // DB is an interface describing a persisted Lightning Network graph.