renew_leases_on_server called with the wrong kind of server

renew_leases_on_server expects a ZKAPAuthorizerStorageClient for the server parameter. However, renew_leases calls it with a NativeStorageServer.

NativeStorageServer.get_storage_server will return the ZKAPAuthorizerStorageClient, though possibly the code also makes the mistake of thinking that every storage server connected will be one of ours.