Progress option of Batch API

[bwbroersma]

Let's first wait for the impact of using the progress in the Batch API.

• Batch API progress data Internet.nl#1506

If it's low impact, the API can be used to report the progress back to the user.
Pipe dream would be to have a progress graph with the domains / time plotted over time and the estimate based on last X measurements.
Re-fetching the status can be done based on the progress, e.g. with a minimal of one per hour, and a maximum of one per minute. This could remove a lot of API calls (although heavier for sure) and give more progress information to the user.