Merge fileCache and dirCache into one single cache in LocalMetadataStore

  /** Contains directories and files. */
  private Cache<Path, PathMetadata> fileCache;

  /** Contains directory listings. */
  private Cache<Path, DirListingMetadata> dirCache;

  static class LocalMetaEntry {
    @Nullable
    public PathMetadata pathMetadata;
    @Nullable
    public DirListingMetadata dirListingMetadata;
  }

Pair<PathMetadata, DirListingMetadata> metaEntry;