Simplify session cleanup related scheduled jobs

[vpavic]

At present, RedisIndexedHttpSessionConfiguration and JdbcHttpSessionConfiguration rely on @EnableScheduling annotated inner configuration classes to set up their session cleanup related scheduled jobs.

This feels a bit invasive, as it silently opts in users into a general purpose task scheduling support provided by Spring Framework. In reality, those session cleanup jobs should really need a single thread that's dedicated to their execution and does not eat into the thread pool used by general purpose task scheduling (and vice-versa actually).

We should look into simplifying this setup, so that SessionRepository implementations that required session cleanup jobs manage their own scheduled executor service with minimal resource footprint.