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..950c3ca1900029b835aa219648565b7790551e60 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 TimerResult
 
 	// Reset configures the timer to expire after a specified duration.
 	//
@@ -29,3 +28,16 @@ type Timer interface {
 	// If the timer is active, it will be stopped and this method will return true.
 	Stop() bool
 }
+
+// TimerResult is the result fo a timer operation.
+//
+// If the timer operation completed successfully, the embedded time.Time will be
+// set to the time when the timer triggered. If the timer was prematurely
+// terminated, Err will be non-nil.
+type TimerResult struct {
+	time.Time
+
+	// Err, if not nil, indicates that After did not finish naturally and contains
+	// the reason why.
+	Err error
+}