pull: Add queuing into the higher level logic

pull: Add queuing into the higher level logic

Working on the libcurl backend, I didn't want to reimplement another queue. I
think the queue logic is really better done at the high level, since the fetcher
knows how we want to prioritize metadata over content, etc.

Adding another queue here is duplication, but things will look nicer when we can
actually delete the libsoup one in the next commit.

Closes: #654
Approved by: jlebon