multi: further decouple graph

To further separate the channel graph from the channel state, we
refactor the AddrsForNode method to use the graphs's public methods
instead of directly accessing any buckets. This makes sure that we can
have the channel state cached with just its buckets while not using a
kvdb level cache for the graph.
At the same time we refactor the graph's test to also be less dependent
upon the channel state DB.

autopilot/graph.go

@@ -148,7 +148,7 @@ func (d *databaseChannelGraph) addRandChannel(node1, node2 *btcec.PublicKey,
 				return nil, err
 			}
 
-			dbNode, err := d.db.FetchLightningNode(nil, vertex)
+			dbNode, err := d.db.FetchLightningNode(vertex)
 			switch {
 			case err == channeldb.ErrGraphNodeNotFound:
 				fallthrough