zodb/go: In-RAM client cache

The cache is needed so that we can provide IStorage.Prefetch
functionality generally wrapped on top of a storage driver: when an
object is loaded, the loading itself consists of steps:

1. start loading object into cache,
2. wait for the loading to complete.

This way Prefetch is naturally only "1" - start loading object into
cache but do not wait for the loading to be complete. Go's goroutines
naturally help here where we can spawn every such loading into its own
goroutine instead of explicitly programming loading in terms of a state
machine.

Since this cache is mainly needed for Prefetch to work, not to actually
cache data (though it works as cache for repeating access too), the goal
when writing it was to add minimal overhead for "data-not-yet-in-cache"
case. Current state we are not completely there yet but the latency is
acceptable - depending on the workload the cache layer adds ~

	0.5 - 1 - 3µs

to loading times.