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/snapper/manual.go

@@ -0,0 +1,15 @@
+package snapper
+
+import (
+	"context"
+)
+
+type manual struct{}
+
+func (s *manual) Run(ctx context.Context, wakeUpCommon chan<- struct{}) {
+	// nothing to do
+}
+
+func (s *manual) Report() Report {
+	return Report{Type: TypeManual, Manual: &struct{}{}}
+}