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.