Implement equals and hashCode in Chunk class

See https://github.com/spring-projects/spring-batch/issues/4296. There is a problem with the PR https://github.com/spring-projects/spring-batch/pull/4300 that made it into 5.0.1:
it adds equals and hashCode to the ChunkIterator class instead of to the Chunk class.