Refactor MapReduce to use configurer style API

Changes in https://github.com/caskdata/cdap/pull/546

Agree with the review of the MapReduce API. However would like to have this ticket only associates with the change to configurer style and refactoring will be a follow up ticket to keep the change set small.

beforeSubmit and onFinish run inside same transaction as MapReduce

Oh, I just found out that this was changed recently. Now they are in separate transactions.

Still, I believe there's room for improvement of the API.

I'd recommend to review the whole MR APIs to make them better. E.g. we want to address (apart from other things):

beforeSubmit and onFinish run inside same transaction as MapReduce. It is not suitable to do changes in datasets on mapreduce failure (e.g. write some metadata somewhere that idicates about failure) because any changes made to transactional resources (e.g. datasets) will not be visible.