Enqueue codec_conversion transcodes on slower queue

[chvp]

Fixes #653.

[robbevp]

I'd even move this to our slowest queue. If we see these queues as a contrqct wehere the task should be picked up in this time. This will probably not happen when we enqueue these tasks in bulk.

[chvp]

In practice there is little difference; both queues are not used by any other jobs. But agree that the semantics of whenever are clearer for this case.