tun: Read the clock once per staged loop iteration

The TX reader timed its slot wait with a second pair of clock reads only to
subtract it again. It now waits for a free slot before it starts the busy
clock, so a read costs one time.Now and one time.Since. The RX listener
likewise ends its busy time when it has to wait for a batch instead of
timing the wait, and pool workers time a run of queued jobs with one pair
of reads. A test checks the read stages still leave backpressure out.

flow.go

@@ -108,8 +108,12 @@ func newFlowPool(workers, queueLen int, busy metrics.Counter) *flowPool {
 		p.wg.Go(func() {
 			nb := make([]byte, 12)
 			for job := range p.jobs {
+				// One pair of clock reads covers a run of jobs that are queued back to back
 				t := time.Now()
-				job.run(nb)
+				for job != nil {
+					job.run(nb)
+					job = p.next()
+				}
 				busy.Inc(int64(time.Since(t)))
 			}
 		})
@@ -117,6 +121,16 @@ func newFlowPool(workers, queueLen int, busy metrics.Counter) *flowPool {
 	return p
 }
 
+// next returns the next queued job without waiting, or nil.
+func (p *flowPool) next() flowJob {
+	select {
+	case job := <-p.jobs:
+		return job
+	default:
+		return nil
+	}
+}
+
 func (p *flowPool) close() { close(p.jobs); p.wg.Wait() }
 
 // stageBusy is the busy time counter of one stage of routine q.