routing: fix panic in inner loop of path finding

This commit seems to fix a sporadic error within the integration tests
which would at times cause a panic when a payment as initiated.

This issue was with the way were deleting from the middle of the slice
of unvisited nodes within the graph. Assigning the last element to the
middle would at times cause a panic the last element may be nil. To fix
this, we now manually copy every item over by one, preserving the order
of the slice, and possibly fixing the panic once and for all.

routing/pathfind.go

@@ -285,7 +285,7 @@ func findRoute(graph *channeldb.ChannelGraph, target *btcec.PublicKey,
 
 				// Since we're going to visit this node, we can
 				// remove it from the set of unvisited nodes.
-				unvisited[i] = unvisited[len(unvisited)-1]
+				copy(unvisited[i:], unvisited[i+1:])
 				unvisited[len(unvisited)-1] = nil // Avoid GC leak.
 				unvisited = unvisited[:len(unvisited)-1]