fix addurl concurrency issue

fix addurl concurrency issue

addurl: Support adding the same url to multiple files at the same time when
using -J with --batch --with-files.

Implementation was easier than expected, was able to reuse OnlyActionOn.

While it will download the url's content multiple times, that seems like
the best thing to do; see my comment for why.

Sponsored-by: Dartmouth College's DANDI project