This caches all the objects returned from the List call. This makes
opening them much quicker so speeds up prune and restores. It also
uses fewer transactions. It can be disabled with
`--cache-objects=false`.
This was discovered when using the B2 backend when the budget was
being blown on list object calls which can avoided with a bit of
caching.
For typical 1 million file backup for a latop or server this will only
use a small amount more memory.
