Add Context cancellation to clock.

common/clock/external.go

Index: common/clock/external.go
diff --git a/common/clock/external.go b/common/clock/external.go
index acca11b0cdc651295b391da3426ffb8c35655e66..d564b101822b0cdb666d2595b3eac689d3212ae3 100644
--- a/common/clock/external.go
+++ b/common/clock/external.go
@@ -50,16 +50,16 @@ func Now(ctx context.Context) time.Time {
 
 // Sleep calls Clock.Sleep on the Clock instance stored in the supplied Context.
 func Sleep(ctx context.Context, d time.Duration) {
-	Get(ctx).Sleep(d)
+	Get(ctx).Sleep(ctx, d)
 }
 
 // NewTimer calls Clock.NewTimer on the Clock instance stored in the supplied
 // Context.
 func NewTimer(ctx context.Context) Timer {
-	return Get(ctx).NewTimer()
+	return Get(ctx).NewTimer(ctx)
 }
 
 // After calls Clock.After on the Clock instance stored in the supplied Context.
-func After(ctx context.Context, d time.Duration) <-chan time.Time {
-	return Get(ctx).After(d)
+func After(ctx context.Context, d time.Duration) <-chan TimerResult {
+	return Get(ctx).After(ctx, d)
 }