Sketch a scheduler for running periodic tasks

Periodic tasks are special in that they are usually kept around and
started based on a schedule. A new component checks periodic tasks and
submits them in the queue once they are due.

In order to avoid duplicate periodic jobs, the tracker of a job is
used to store the periodic job id. Each time a periodic task is due,
it is first checked if there is a job running (or queued) for this
task.

modules/store/src/main/resources/db/migration/mariadb/V1.3.0__periodic_job.sql

@@ -0,0 +1,15 @@
+CREATE TABLE `periodic_task` (
+  `id` varchar(254) not null primary key,
+  `enabled` boolean not null,
+  `task` varchar(254) not null,
+  `group_` varchar(254) not null,
+  `args` text not null,
+  `subject` varchar(254) not null,
+  `submitter` varchar(254) not null,
+  `priority` int not null,
+  `worker` varchar(254),
+  `marked` timestamp,
+  `timer` varchar(254) not null,
+  `nextrun` timestamp not null,
+  `created` timestamp not null
+);