zodb/go provides generic cache (see 7233b4c0 "zodb/go: In-RAM client
cache") primarily in order for prefetch to work. However if we need to
benchmark a storage with loading some objects several times, this cache
can hide the actual time it takes for an object to load.

For such use cases add NoCache open option so that opening does not
create a cache and always conveys load operations directly to storage
driver. The option will be used by zwrk tool (see next patch).