Improve start up of scheduler background thread

[neshanjo]

Currently, the background thread is started by a GET on /api/wakeup in a ServletContextListener (see BackgroundTaskStarter). It is done this way in order to have EntityManagerFactory and SwitchControl injected. However, this is more a workaround than an elegant solution.

(A non-solution for me would be to write a separate instantiation for EntityManagerFactory and SwitchControl - the instantiation should happen in one place only!)

Possible more elegant ways:

• Find a way of using jersey's dependency injector to inject these dependencies directly into BackgroundTaskStarter (I haven't found any...)
• Use a different dependency injection framework that is available both in REST resource classes and servlet context listeners