Add Context cancellation to clock.

common/clock/clockcontext.go

Index: common/clock/clockcontext.go
diff --git a/common/clock/clockcontext.go b/common/clock/clockcontext.go
index 979f2fc0588d02a29fc808553ff43742c9dbb640..1b9df12e35e83e0a6be23c9ece273f3e7f04fafd 100644
--- a/common/clock/clockcontext.go
+++ b/common/clock/clockcontext.go
@@ -71,11 +71,8 @@ func WithDeadline(parent context.Context, deadline time.Time) (context.Context,
 
 	// Invoke our cancelFunc after the specified time.
 	go func() {
-		select {
-		case <-c.Done():
-			break
-
-		case <-After(c, d):
+		if ar := <-After(c, d); ar.Err == nil {
+			// Timer expired naturally.
 			c.setError(context.DeadlineExceeded)
 			cancelFunc()
 		}