sheep/http: use a dynamic buffer size for read/write buffer allocation

Allocate the buffer size based on the request length so that small request won't
take too much memory.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>