Revisit the java DSL for job flow definition

[fmbenhassine]

Compared to its quite stable XML counterpart, the java DSL for job flow definition has multiple issues: https://github.com/spring-projects/spring-batch/issues?q=is%3Aissue+is%3Aopen+label%3A%22related-to%3A+flow-definition%22

We need to revisit this in v6.