graph/db: move cache read checks to ChannelGraph.

This commit moves the graph cache checks for FetchNodeFeatures,
ForEachNodeDirectedChannel, GraphSession and ForEachNodeCached from the
KVStore to the ChannelGraph. Since the ChannelGraph is currently just a
pass-through for any of the KVStore methods, all that needs to be done
for calls to go via the ChannelGraph instead directly to the KVStore is
for the ChannelGraph to go and implement those methods.

graph/db/graph_test.go

@@ -3953,7 +3953,7 @@ func TestGraphCacheForEachNodeChannel(t *testing.T) {
 
 	getSingleChannel := func() *DirectedChannel {
 		var ch *DirectedChannel
-		err = graph.forEachNodeDirectedChannel(nil, node1.PubKeyBytes,
+		err = graph.ForEachNodeDirectedChannel(node1.PubKeyBytes,
 			func(c *DirectedChannel) error {
 				require.Nil(t, ch)
 				ch = c