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.

autopilot/prefattach.go

@@ -1,6 +1,7 @@
 package autopilot
 
 import (
+	"context"
 	prand "math/rand"
 	"time"
 
@@ -82,14 +83,18 @@ func (p *PrefAttachment) NodeScores(g ChannelGraph, chans []LocalChannel,
 	chanSize btcutil.Amount, nodes map[NodeID]struct{}) (
 	map[NodeID]*NodeScore, error) {
 
+	ctx := context.TODO()
+
 	// We first run though the graph once in order to find the median
 	// channel size.
 	var (
 		allChans  []btcutil.Amount
 		seenChans = make(map[uint64]struct{})
 	)
-	if err := g.ForEachNode(func(n Node) error {
-		err := n.ForEachChannel(func(e ChannelEdge) error {
+	if err := g.ForEachNode(ctx, func(ctx context.Context, n Node) error {
+		err := n.ForEachChannel(ctx, func(_ context.Context,
+			e ChannelEdge) error {
+
 			if _, ok := seenChans[e.ChanID.ToUint64()]; ok {
 				return nil
 			}
@@ -114,15 +119,19 @@ func (p *PrefAttachment) NodeScores(g ChannelGraph, chans []LocalChannel,
 	// the graph.
 	var maxChans int
 	nodeChanNum := make(map[NodeID]int)
-	if err := g.ForEachNode(func(n Node) error {
+	if err := g.ForEachNode(ctx, func(ctx context.Context, n Node) error {
 		var nodeChans int
-		err := n.ForEachChannel(func(e ChannelEdge) error {
+		err := n.ForEachChannel(ctx, func(_ context.Context,
+			e ChannelEdge) error {
+
 			// Since connecting to nodes with a lot of small
 			// channels actually worsens our connectivity in the
 			// graph (we will potentially waste time trying to use
 			// these useless channels in path finding), we decrease
 			// the counter for such channels.
-			if e.Capacity < medianChanSize/minMedianChanSizeFraction {
+			if e.Capacity <
+				medianChanSize/minMedianChanSizeFraction {
+
 				nodeChans--
 				return nil
 			}