rake kithe:create_derivatives gets some options for creating Background Activejob

[jrochkind]

We already had a kithe:create_derivatives task with many options for different things you'd want to do for creating derivatives -- but it did it all in the foreground, serially, in a single thread.

I had need to instead create many small background jobs (which we can then easily handle on heroku by scaling up workers, to get it done quickly!). And realized small changes to the existing CreateDerivativesJob and the rake task would make that possible.

So do so here.

For the thing we want to do in our app, for instance, we can run (on heroku even)

heroku run "rake kithe:create_derivatives -- --derivatives=graphiconly_pdf --lazy --bg=special_jobs"

That will create one BG job per Asset (for all assets!), in the ActiveJob queue special_jobs. The job will be queued up for every Asset, but since we said "lazy", it will only actually create the graphiconly_pdf derivative if one doesn't already exist, otherwise the job will just exit.

[jrochkind]

No tests here, this stuff doesn't have GREAT ways to test and lacked existing tests.

I did some tests manually.

Eh, maybe I'll think about adding at least a bit of tests....

[jrochkind]

Nah, I'm not going to do tests right now.

[jrochkind]

OK I do need tests, cause I had a bug ha!