Add Context cancellation to clock.

common/parallel/semaphore.go

Index: common/parallel/semaphore.go
diff --git a/common/parallel/semaphore.go b/common/parallel/semaphore.go
index c2ddafb80a5839745b1ceb6eadb88fed07f36f84..f8299a59b0f0f721e2d22219d4ce6b3011df83df 100644
--- a/common/parallel/semaphore.go
+++ b/common/parallel/semaphore.go
@@ -25,3 +25,11 @@ func (s Semaphore) Unlock() {
 		<-s
 	}
 }
+
+// TakeAll blocks until it holds all available semaphore resources. When it
+// returns, the caller owns all of the resources in the semaphore.
+func (s Semaphore) TakeAll() {

[dnj (Google), 2016/02/10 03:19:04]

This is really useful. Before, one would have to use a WaitGroup in parallel to
a Semaphore to Join against it. Now it's a single command! Needed b/c ackbuffer
was using WaitGroup.

+	for i := 0; i < cap(s); i++ {
+		s.Lock()
+	}
+}