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)

daemon/daemon.go

@@ -176,6 +176,18 @@ func (s *jobs) reset(job string) error {
 	return wu()
 }
 
+func (s *jobs) setConcurrency(jobName string, concurrency int) error {
+	s.m.RLock()
+	defer s.m.RUnlock()
+
+	job, ok := s.jobs[jobName]
+	if !ok {
+		return errors.Errorf("Job %q does not exist", job)
+	}
+
+	return job.SetConcurrency(concurrency)
+}
+
 const (
 	jobNamePrometheus = "_prometheus"
 	jobNameControl    = "_control"