autopilot: start threading contexts through

The `GraphSource` interface in the `autopilot` package is directly
implemented by the `graphdb.KVStore` and so we will eventually thread
contexts through to this interface. So in this commit, we start updating
the autopilot system to thread contexts through in preparation for
passing the context through to any calls made to the GraphSource.

Two context.TODOs are added here which will be addressed in follow up
commits.

discovery/bootstrapper.go

@@ -161,6 +161,8 @@ func (c *ChannelGraphBootstrapper) SampleNodeAddrs(_ context.Context,
 	numAddrs uint32,
 	ignore map[autopilot.NodeID]struct{}) ([]*lnwire.NetAddress, error) {
 
+	ctx := context.TODO()
+
 	// We'll merge the ignore map with our currently selected map in order
 	// to ensure we don't return any duplicate nodes.
 	for n := range ignore {
@@ -183,7 +185,9 @@ func (c *ChannelGraphBootstrapper) SampleNodeAddrs(_ context.Context,
 			errFound = fmt.Errorf("found node")
 		)
 
-		err := c.chanGraph.ForEachNode(func(node autopilot.Node) error {
+		err := c.chanGraph.ForEachNode(ctx, func(_ context.Context,
+			node autopilot.Node) error {
+
 			nID := autopilot.NodeID(node.PubKey())
 			if _, ok := c.tried[nID]; ok {
 				return nil