message: [storage] set default prune window
  On test net our full nodes exhausted disk in roughly a week's time.
  Setting a default prune window to prevent that. (40Mil is >4 days
  worth of history assuming we have 100 tps on avg.)
  Closes:
  Approved by: sausagee
diff: @@ -26,7 +26,9 @@ impl Default for StorageConfig {
  backup_service_port: 7777,
  dir: PathBuf::from("libradb/db"),
  grpc_max_receive_len: Some(100_000_000),
  - prune_window: None,
  + // At 100 tps on avg, we keep 4~5 days of history.
  + // n.b. Validators have more aggressive override in the config builder.
  + prune_window: Some(40_000_000),
  data_dir: PathBuf::from("/opt/libra/data/common"),
  }
  }
  