Add Context cancellation to clock.

common/clock/timer.go

Index: common/clock/timer.go
diff --git a/common/clock/timer.go b/common/clock/timer.go
index aa03c383dda4f710b8c1745d62fca5d645a9f1c8..9e2853fb21f4d7baad58b9c7a354b43655ec27db 100644
--- a/common/clock/timer.go
+++ b/common/clock/timer.go
@@ -13,9 +13,8 @@ import (
 // A Timer is instantiated from a Clock instance and started via its Reset()
 // method.
 type Timer interface {
-	// GetC returns the underlying timer's channel, or nil if the timer is no
-	// running.
-	GetC() <-chan time.Time
+	// GetC returns the underlying timer's channel.
+	GetC() <-chan AfterResult
 
 	// Reset configures the timer to expire after a specified duration.
 	//