fetcher: Avoid too large queues for metadata processing

fetcher: Avoid too large queues for metadata processing

We added backoff/queueing for fetching via HTTP, but we have
another queue in the metadata scanning which can also grow
up to the number of outstanding objects, which can be large.

Capping the scanning operation when we have hit our operation
limit will avoid potentially large amounts of allocations in the
case of e.g. a slow network.

Closes: https://github.com/ostreedev/ostree/issues/2732