refactor snapper & support cron-based snapshotting

fixes https://github.com/zrepl/zrepl/issues/554
refs https://github.com/zrepl/zrepl/discussions/547#discussioncomment-1936126

daemon/job/active.go

@@ -101,7 +101,7 @@ type modePush struct {
 	receiver      *rpc.Client
 	senderConfig  *endpoint.SenderConfig
 	plannerPolicy *logic.PlannerPolicy
-	snapper       *snapper.PeriodicOrManual
+	snapper       snapper.Snapper
 }
 
 func (m *modePush) ConnectEndpoints(ctx context.Context, connecter transport.Connecter) {
@@ -137,7 +137,8 @@ func (m *modePush) RunPeriodic(ctx context.Context, wakeUpCommon chan<- struct{}
 }
 
 func (m *modePush) SnapperReport() *snapper.Report {
-	return m.snapper.Report()
+	r := m.snapper.Report()
+	return &r
 }
 
 func (m *modePush) ResetConnectBackoff() {

daemon/job/passive.go

@@ -58,7 +58,7 @@ func modeSinkFromConfig(g *config.Global, in *config.SinkJob, jobID endpoint.Job
 
 type modeSource struct {
 	senderConfig *endpoint.SenderConfig
-	snapper      *snapper.PeriodicOrManual
+	snapper      snapper.Snapper
 }
 
 func modeSourceFromConfig(g *config.Global, in *config.SourceJob, jobID endpoint.JobID) (m *modeSource, err error) {
@@ -88,7 +88,8 @@ func (m *modeSource) RunPeriodic(ctx context.Context) {
 }
 
 func (m *modeSource) SnapperReport() *snapper.Report {
-	return m.snapper.Report()
+	r := m.snapper.Report()
+	return &r
 }
 
 func passiveSideFromConfig(g *config.Global, in *config.PassiveJob, configJob interface{}, parseFlags config.ParseFlags) (s *PassiveSide, err error) {

daemon/job/snapjob.go

@@ -26,7 +26,7 @@ import (
 type SnapJob struct {
 	name     endpoint.JobID
 	fsfilter zfs.DatasetFilter
-	snapper  *snapper.PeriodicOrManual
+	snapper  snapper.Snapper
 
 	prunerFactory *pruner.LocalPrunerFactory
 
@@ -86,7 +86,8 @@ func (j *SnapJob) Status() *Status {
 		s.Pruning = j.pruner.Report()
 	}
 	j.prunerMtx.Unlock()
-	s.Snapshotting = j.snapper.Report()
+	r := j.snapper.Report()
+	s.Snapshotting = &r
 	return &Status{Type: t, JobSpecific: s}
 }