Fix stuck acquire queues on partial server errors

When a server responds with two InRelease files, but one
is good, and the other indicates a temporary error, we
queued the other indices from the good repository for
downloading and then queued the retry.

The resulting queue ended up with items having fetchAfter=0
before the item with fetchAfter=<something>, causing the
Run() loop to not detect a stuck queue.

Change the order of the queue such that the highest retry-after
items comes first; this ensures that we always see stuck queues.

This of course changes the behavior of retries in that if one
file fails temporarily we block the entire server. This does
seem more beneficial in the common case - if one file fails,
probably all of them fail, and there's no point bombarding
the server with requests for indices from good repositories
until all have failed.

LP: #2003851