Before we were inserting the id of the cover into the registry
from a rayon thread. But rayon will only execute N threads at the
same time and let the rest into a queue. This would casue mutliple
jobs being queued since the cover id was not inserted in the
registry until the downloading had started.
This fixes said behavior by having the main thread block and write
in the id in the registry.
993b6e9d0a