manifest: make manifestlog a storecache The old @property on manifestlog was broken. It meant that we would always recreate the manifestlog instance, which meant the cache was never hit. Since we'll eventually remove repo.manifest and make manifestlog the only property, let's go ahead and make manifestlog the @storecache property, have manifestlog own the manifest instance, and have repo.manifest refer to it via manifestlog. This means all accesses go through repo.manifestlog, which is now invalidated correctly.