add option to disable step holds for incremental sends

This is a stop-gap solution until we re-write the pruner to support
rules for removing step holds.

Note that disabling step holds for incremental sends does not affect
zrepl's guarantee that incremental replication is always possible:

Suppose you yank the external drive during an incremental @from -> @to step:

* restarting that step or future incrementals @from -> @to_later` will be possible
  because the replication cursor bookmark points to @from until the step is complete
* resuming @from -> @to will work as long as the pruner on your internal pool doesn't come around to destroy @to.
    * in that case, the replication algorithm should determine that the resumable state
      on the receiving side isuseless because @to no longer exists on the sending side,
      and consequently clear it, and restart an incremental step @from -> @to_later

refs #288

daemon/job/active.go

@@ -152,9 +152,10 @@ func modePushFromConfig(g *config.Global, in *config.PushJob, jobID endpoint.Job
 	}
 
 	m.senderConfig = &endpoint.SenderConfig{
-		FSF:     fsf,
-		Encrypt: &zfs.NilBool{B: in.Send.Encrypted},
-		JobID:   jobID,
+		FSF:                         fsf,
+		Encrypt:                     &zfs.NilBool{B: in.Send.Encrypted},
+		DisableIncrementalStepHolds: in.Send.StepHolds.DisableIncremental,
+		JobID:                       jobID,
 	}
 	m.plannerPolicy = &logic.PlannerPolicy{
 		EncryptedSend: logic.TriFromBool(in.Send.Encrypted),

daemon/job/passive.go

@@ -79,9 +79,10 @@ func modeSourceFromConfig(g *config.Global, in *config.SourceJob, jobID endpoint
 		return nil, errors.Wrap(err, "cannot build filesystem filter")
 	}
 	m.senderConfig = &endpoint.SenderConfig{
-		FSF:     fsf,
-		Encrypt: &zfs.NilBool{B: in.Send.Encrypted},
-		JobID:   jobID,
+		FSF:                         fsf,
+		Encrypt:                     &zfs.NilBool{B: in.Send.Encrypted},
+		DisableIncrementalStepHolds: in.Send.StepHolds.DisableIncremental,
+		JobID:                       jobID,
 	}
 
 	if m.snapper, err = snapper.FromConfig(g, fsf, in.Snapshotting); err != nil {

daemon/job/snapjob.go

@@ -175,6 +175,8 @@ func (j *SnapJob) doPrune(ctx context.Context) {
 		FSF:   j.fsfilter,
 		// FIXME encryption setting is irrelevant for SnapJob because the endpoint is only used as pruner.Target
 		Encrypt: &zfs.NilBool{B: true},
+		// FIXME DisableIncrementalStepHolds setting is irrelevant for SnapJob because the endpoint is only used as pruner.Target
+		DisableIncrementalStepHolds: false,
 	})
 	j.pruner = j.prunerFactory.BuildLocalPruner(ctx, sender, alwaysUpToDateReplicationCursorHistory{sender})
 	log.Info("start pruning")