func ParseStepHoldTag(tag string) (JobID, error) {
	match := stepHoldTagRE.FindStringSubmatch(tag)
	if match == nil {
		return JobID{}, fmt.Errorf("parse hold tag: match regex %q", stepHoldTagRE)
	}
	jobID, err := MakeJobID(match[1])
	if err != nil {
		return JobID{}, errors.Wrap(err, "parse hold tag: invalid job id field")
	}
	return jobID, nil
}
err != nil always means that the bookmark is not a step bookmark
package endpoint
import (
	"context"
	"fmt"
	"regexp"
	"github.com/pkg/errors"
	"github.com/zrepl/zrepl/zfs"
)
var stepHoldTagRE = regexp.MustCompile("^zrepl_STEP_J_(.+)")
func StepHoldTag(jobid JobID) (string, error) {
	return stepHoldTagImpl(jobid.String())
}
func stepHoldTagImpl(jobid string) (string, error) {
	t := fmt.Sprintf("zrepl_STEP_J_%s", jobid)
	if err := zfs.ValidHoldTag(t); err != nil {
		return "", err
	}
	return t, nil
}