BATCH-1994: Add protected DAO getters to SimpleJobExplorer

Added getters for DAOs used in SimpleJobExplorer, to allow the class to be extended more easily.

Since I'm trying to get a feel of the development process with my first PR to Spring Batch, I might be wrong in my assumption to make this change off of the 2.1.9 tag and make a PR to 2.1.x only, for this ticket with an affected version of 2.1.9. Please let me know if this is not the correct approach. Thanks.