整理代码风格并修复若干bug

1. fixed coding style, remove some redundant code
2. bugfix: read large/custom file timeout or core
3. bugfix: close file may failed (crc error) when retry occurs
4. bugfix: should not select data server when invalid local block hit
5. bugfix: should not return 500 when client time out
6. bugfix: should parse status msg instead of read response msg when ds can not serve
7. bugfix: modify default body buffer size for recv from upstream to 2M
8. bugfix: some wrong return code
9. bugfix: fix read corrupted large file bug
10.new feature: add last-modified header in response if possible