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/joex/src/main/resources/reference.conf

@@ -64,6 +64,19 @@ docspell.joex {
     wakeup-period = "30 minutes"
   }
 
+  periodic-scheduler {
+
+    # Each scheduler needs a unique name. This defaults to the node
+    # name, which must be unique, too.
+    name = ${docspell.joex.app-id}
+
+    # A fallback to start looking for due periodic tasks regularily.
+    # Usually joex instances should be notified via REST calls if
+    # external processes change tasks. But these requests may get
+    # lost.
+    wakeup-period = "10 minutes"
+  }
+
   # Configuration of text extraction
   extraction {
     # For PDF files it is first tried to read the text parts of the