code refactor about purge and rewrite

[hicqu]

Signed-off-by: qupeng qupeng@pingcap.com

[Little-Wallace]

Could we split the compression from LogBatch? And we can encode the data when put any new item. Just like this:

struct LogBatch{
    items: Vec<LogItem>,
    data: Vec<u8>,
}

impl LogBatch {
      fn put(&mut self, entries: Vec<Entry>) {
           let item = LogItem::Entries(entries);
           item.encode_to(&mut self.data);
           self.items.push(item);
      }
     fn data(&self) -> &[u8] {
          &self.data
     }
}

impl PipeLog {
         fn write<E: Message, W: EntryExt<E>>(
        &self,
        data: &[u8],
        mut sync: bool,
    ) -> Result<(usize,usize, Option<CacheTracker)> {
         if data.len() > self.compression_threshold {
            let compress_data = lz4::encode_block(data, HEADER_LEN);
            self.append(LogQueue::Append, &compress_data, &mut sync)?
         }  else {
            self.append(LogQueue::Append, &data, &mut sync)?
         }
        .....
    }
}

[Little-Wallace]

And we do not need to store compression type in EntryIndex. Because we must read the data from file at first, we can judge whether this data has been compressed by HEADER.