Introduce multiple-warcs-per-job

[tokee]

Due to an internal project, it would help to have the option of starting jobs from arctica with more than 1 WARC as argument. This would also help in cases with very small WARCs to lessen the amount of overhead for job starts.

The tricky part is how to handle failure of one of the WARCs in a batch. Ideally only that WARC would be treated as problematic.