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)

client/signal.go

@@ -9,7 +9,7 @@ import (
 )
 
 var SignalCmd = &cli.Subcommand{
-	Use:   "signal [wakeup|reset] JOB",
+	Use:   "signal [wakeup|reset] JOB [DATA]",
 	Short: "wake up a job from wait state or abort its current invocation",
 	Run: func(subcommand *cli.Subcommand, args []string) error {
 		return runSignalCmd(subcommand.Config(), args)
@@ -17,8 +17,13 @@ var SignalCmd = &cli.Subcommand{
 }
 
 func runSignalCmd(config *config.Config, args []string) error {
-	if len(args) != 2 {
-		return errors.Errorf("Expected 2 arguments: [wakeup|reset] JOB")
+	if len(args) < 2 || len(args) > 3 {
+		return errors.Errorf("Expected 2 arguments: [wakeup|reset|set-concurrency] JOB [DATA]")
+	}
+
+	var data string
+	if len(args) == 3 {
+		data = args[2]
 	}
 
 	httpc, err := controlHttpClient(config.Global.Control.SockPath)
@@ -30,9 +35,11 @@ func runSignalCmd(config *config.Config, args []string) error {
 		struct {
 			Name string
 			Op   string
+			Data string
 		}{
 			Name: args[1],
 			Op:   args[0],
+			Data: data,
 		},
 		struct{}{},
 	)