Implement very basic grouped fetches of objects from the cloud.

Add an item prefetch call, which for now just collects together items that
have been prefetched and when one of them is actually fetched, a range
request is sent that will cover all the objects.  Thus this doesn't improve
the time to fetch the first object (actually it will hurt), it does arrange
to have objects fetched in groups where possible which reduces the total
number of operations and the time to get subsequent objects.