Provide a PruneUnreachableObjects RPC to replace GarbageCollect

Rails sometimes needs to ask Gitaly to prune all unreachable objects, most importantly in the context where a user wants to remove accidentally-committed data from a repository. The current way to do this is to call GarbageCollect() with prune = true, but that RPC also controls other parameters like writing bitmaps. This is control we don't want to give callers, so we should instead provide a new RPC whose only task is to prune unreachable objects.