Allow override factory properties in user-defined configurer subclass

[andreysaksonov]

E.g. to set custom serializer. It was a pain to workaround https://jira.spring.io/browse/BATCH-2575 in spring-batch 3.x because neither factories nor dataSource/transactionManager are not exposed to subclass of default configurer - so the only solution is to create new implementation of BatchConfigurer and copy-paste most of the DefaultBatchConfigurer code there just to call one setter on factory.

[mminella]

I think this takes the wrong approach. We really should enable sub classes better access to the DataSource and TransactionManager involved instead of exposing factory beans like this. There is a pull request that already does that in which we are currently looking at bringing in (in some form) for 4.1: #415

[andreysaksonov]

@mminella The idea was to use exposed methods first in subclass (super.createFooFactory) first, then apply additional configurations on the factory (call .setSomeProperty on factory), then DefaultConfigurer will use customized factories to create actual instance of repository/explorer. Exposure of transactionManager/dataSource will require to copy-paste again creation of factories/beans in subclass.
However, this PR is just simple solution aimed not to touch too many code. The high level issue is that there is only single configurer for everything, but you may want to customize only single bean and use defaults for others.