a8n: Enqueue CampaignJobs in goroutine to not block request

Created by: mrnugget

This was an oversight on my part when doing #6598. (I think I failed a lot of "Concurrency in Go" tests in the past 24 hours.)

Since #6598 we have a fixed number of workers working off CampaignJobs. We only want to wait for their completion by calling (&Runner).Wait.

The previous version of the code here, though, blocked until all jobs were enqueued. And since the queue is an unbuffered channel, that might take a while since we only ever work off $num-of-workers at the same time.

This fixes what @eseliger reported in Slack