WIP runtime-controllable concurrency for replication

Changes done so far:
- signal to route concurrency request up to the stepQueue
    - pretty hacky, no status reporting yet
- stepQueue upsizing (confirmed that it works, no intermediary commit)

Stuck at: stepQueue downsizing
- idea was to have the stepQueue signal to the activated step that it
  should suspend
- ideally, we'd just kill everything associated with the step and track
  it as restartable
    - the driver model doesn't allow for that though within an attempt
    - would need to start a separate run for the step
- less perfect: tell the downsized steps to stop copying, but leave
  all zfs sends + rpc conns open
    - - doesn't give back the resoures that a step aquired before
      being selected as downsize victim (open zfs processe + TCP conn,
      some memory in the pipe)
    - - looks weird to user if the ps aux
    - + re-waking a step is easy: just tell it to proceed with copying
    - (the impl would likely pass a check function down into Step.do
      and have it check that functino periodically. the suspend should
      be acknowledged, and stepQueue should only remove the step from
      the active queue _after_ that step has acknowledged that is
      suspended)

replication/driver/replication_stepqueue_test.go

@@ -17,7 +17,7 @@ import (
 // (relies on scheduler responsivity of < 500ms)
 func TestPqNotconcurrent(t *testing.T) {
 	var ctr uint32
-	q := newStepQueue()
+	q := newStepQueue(1)
 	var wg sync.WaitGroup
 	wg.Add(4)
 	go func() {
@@ -29,7 +29,7 @@ func TestPqNotconcurrent(t *testing.T) {
 	}()
 
 	// give goroutine "1" 500ms to enter queue, get the active slot and enter time.Sleep
-	defer q.Start(1)()
+	defer q.Start()()
 	time.Sleep(500 * time.Millisecond)
 
 	// while "1" is still running, queue in "2", "3" and "4"
@@ -77,8 +77,9 @@ func (r record) String() string {
 // Hence, perform some statistics on the wakeup times and assert that the mean wakeup
 // times for each step are close together.
 func TestPqConcurrent(t *testing.T) {
-
-	q := newStepQueue()
+	
+	concurrency := 5
+	q := newStepQueue(concurrency)
 	var wg sync.WaitGroup
 	filesystems := 100
 	stepsPerFS := 20
@@ -104,8 +105,7 @@ func TestPqConcurrent(t *testing.T) {
 			records <- recs
 		}(fs)
 	}
-	concurrency := 5
-	defer q.Start(concurrency)()
+	defer q.Start()()
 	wg.Wait()
 	close(records)
 	t.Logf("loop done")