update_oai-pmh Add a back-off strategy, to allow a service to recover

Motivation:

Some server-side problems are quickly fixed by retrying the request,
while other problems take longer to recover.  Using a fixed duration as
a delay between attempts makes it hard to reconcile these different
failure modes.

Modification:

Add a progressive timeout strategy.  The delay between attempts now
scales linearly between successive attempts.

As a special case, the retry strategy is different if the server has not
provided any useful response.  In this case, a fixed delay is used.

Also, catch more problematic scenarios and handle them with the same
retry strategy.

Result:

More robust handling of server-side errors, by retrying.