channeldb+routing: extend edge lookup methods with zombie index check

In this commit, we extend the graph's FetchChannelEdgesByID and
HasChannelEdge methods to also check the zombie index whenever the edge
to be looked up doesn't exist within the edge index. We do this to
signal to callers that the edge is known, but only as a zombie, and the
only information that we have about the edge are the node public keys of
the two parties involved in the edge.

In the event that an edge does exist within the zombie index, we make
an additional check on edge policies to ensure they are not within the
router's pruning window, indicating that it is a fresh update.

channeldb/error.go

@@ -1,6 +1,9 @@
 package channeldb
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 var (
 	// ErrNoChanDBExists is returned when a channel bucket hasn't been
@@ -79,6 +82,10 @@ var (
 	// can't be found.
 	ErrEdgeNotFound = fmt.Errorf("edge not found")
 
+	// ErrZombieEdge is an error returned when we attempt to look up an edge
+	// but it is marked as a zombie within the zombie index.
+	ErrZombieEdge = errors.New("edge marked as zombie")
+
 	// ErrEdgeAlreadyExist is returned when edge with specific
 	// channel id can't be added because it already exist.
 	ErrEdgeAlreadyExist = fmt.Errorf("edge already exist")