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.

build.sbt

@@ -194,7 +194,8 @@ val store = project.in(file("modules/store")).
       Dependencies.databases ++
       Dependencies.flyway ++
       Dependencies.loggingApi ++
-      Dependencies.emil
+      Dependencies.emil ++
+      Dependencies.calev
   ).dependsOn(common)
 
 val extract = project.in(file("modules/extract")).