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/betweenness_centrality.go

@@ -1,6 +1,7 @@
 package autopilot
 
 import (
+	"context"
 	"fmt"
 	"sync"
 )
@@ -169,7 +170,9 @@ func betweennessCentrality(g *SimpleGraph, s int, centrality []float64) {
 
 // Refresh recalculates and stores centrality values.
 func (bc *BetweennessCentrality) Refresh(graph ChannelGraph) error {
-	cache, err := NewSimpleGraph(graph)
+	ctx := context.TODO()
+
+	cache, err := NewSimpleGraph(ctx, graph)
 	if err != nil {
 		return err
 	}