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/replication.go

@@ -8,6 +8,6 @@ import (
 	"github.com/zrepl/zrepl/replication/driver"
 )
 
-func Do(ctx context.Context, planner driver.Planner) (driver.ReportFunc, driver.WaitFunc) {
-	return driver.Do(ctx, planner)
+func Do(ctx context.Context, initialConcurrency int, planner driver.Planner) (driver.ReportFunc, driver.WaitFunc, driver.SetConcurrencyFunc) {
+	return driver.Do(ctx, initialConcurrency, planner)
 }