fn: cancellations in WaitAsyncResource (#694)

* fn: cancellations in WaitAsyncResource

Added go context with cancel to wait async resource. Although
today, the only case for cancellation is shutdown, this cleans
up agent shutdown a little bit.

* fn: locked broadcast to avoid missed wake-ups

* fn: removed ctx arg to WaitAsyncResource and startDequeuer

This is confusing and unnecessary.

api/agent/async.go

@@ -16,10 +16,13 @@ func (a *agent) asyncDequeue() {
 	defer cancel()
 
 	for {
+		ch, cancelWait := a.resources.WaitAsyncResource()
 		select {
 		case <-a.shutdown:
+			cancelWait()
 			return
-		case <-a.resources.WaitAsyncResource():
+		case <-ch:
+			cancelWait()
 			// TODO we _could_ return a token here to reserve the ram so that there's
 			// not a race between here and Submit but we're single threaded
 			// dequeueing and retries handled gracefully inside of Submit if we run