Implicit count implementation

[etiennebarrie]

We've moved main to prepare a future v2.0.0, mainly by removing deprecated code. One thing we hadn't tackled in order to preserve compatibility is implicit count method for tasks.

If you don't define a count method in your task, we show indeterminate progress in the general case, with exceptions:

• CSV and CSV batches
• Active Record batches

We could extend that, and prevent a lot of implementations of count which are just collection.size. Also that would avoid calling collection twice in a lot of cases (once in build_enumerator and once in count).