treemanifest: cache directory logs and manifests Since manifests instances are cached on the manifest log instance, we can cache directory manifests by caching the directory manifest logs. The directory manifest log cache is a plain dict, so it never expires; we assume that we can keep all the directories in memory. The cache is kept on the root manifestlog, so access to directory manifest logs now has to go through the root manifest log. The caching will soon not be only an optimization. When we start lazily loading directory manifests, we need to make sure we don't create multiple instances of the log objects. The caching takes care of that problem.