snapshotting: support 'periodic' and 'manual' mode

1. Change config format to support multiple types
   of snapshotting modes.
2. Implement a hacky way to support periodic or completely
   manual snaphots.

In manual mode, the user has to trigger replication using the wakeup
mechanism after they took snapshots using their own tooling.

As indicated by the comment, a more general solution would be desirable,
but we want to get the release out and 'manual' mode is a feature that
some people requested...

daemon/snapper/snapper.go

@@ -112,8 +112,8 @@ func getLogger(ctx context.Context) Logger {
 	return logger.NewNullLogger()
 }
 
-func FromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in *config.Snapshotting) (*Snapper, error) {
-	if in.SnapshotPrefix == "" {
+func PeriodicFromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in *config.SnapshottingPeriodic) (*Snapper, error) {
+	if in.Prefix == "" {
 		return nil, errors.New("prefix must not be empty")
 	}
 	if in.Interval <= 0 {
@@ -121,7 +121,7 @@ func FromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in *config.Snap
 	}
 
 	args := args{
-		prefix: in.SnapshotPrefix,
+		prefix: in.Prefix,
 		interval: in.Interval,
 		fsf: fsf,
 		// ctx and log is set in Run()

daemon/snapper/snapper_all.go

@@ -0,0 +1,39 @@
+package snapper
+
+import (
+	"context"
+	"fmt"
+	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/daemon/filters"
+)
+
+// FIXME: properly abstract snapshotting:
+//   - split up things that trigger snapshotting from the mechanism
+//     - timer-based trigger (periodic)
+//     - call from control socket (manual)
+//     - mixed modes?
+//   - support a `zrepl snapshot JOBNAME` subcommand for config.SnapshottingManual
+type PeriodicOrManual struct {
+	s *Snapper
+}
+
+func (s *PeriodicOrManual) Run(ctx context.Context, wakeUpCommon chan <- struct{}) {
+	if s.s != nil {
+		s.s.Run(ctx, wakeUpCommon)
+	}
+}
+
+func FromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in config.SnapshottingEnum) (*PeriodicOrManual, error) {
+	switch v := in.Ret.(type) {
+	case *config.SnapshottingPeriodic:
+		snapper, err := PeriodicFromConfig(g, fsf, v)
+		if err != nil {
+			return nil, err
+		}
+		return &PeriodicOrManual{snapper}, nil
+	case *config.SnapshottingManual:
+		return &PeriodicOrManual{}, nil
+	default:
+		return nil, fmt.Errorf("unknown snapshotting type %T", v)
+	}
+}