message: [backup] trivial: remove magic number 4
  Closes:
  Approved by: wqfish
diff: @@ -7,6 +7,7 @@ use bytes::Bytes;
  use futures::{stream, stream::TryStreamExt};
  use libra_crypto::HashValue;
  use libra_types::{account_state_blob::AccountStateBlob, transaction::Version};
  +use std::mem::size_of;
  use tokio::io::{AsyncRead, AsyncReadExt};
  use tokio_util::compat::FuturesAsyncReadCompatExt;
  @@ -71,7 +72,7 @@ pub async fn backup_account_state(
  let mut state_snapshot_stream = client.get_state_snapshot(version).await?;
  let mut prev_record_bytes: Option<Bytes> = None;
  while let Some(record_bytes) = state_snapshot_stream.read_record_bytes().await? {
  - if chunk.len() + 4 + record_bytes.len() > max_chunk_size {
  + if chunk.len() + size_of::<u32>() + record_bytes.len() > max_chunk_size {
  assert!(chunk.len() <= max_chunk_size);
  let account_state_file = adapter
  .write_new_file(stream::once(async move { chunk }))
  