add method: download_file_to_buff,download_file_to_buff1,
download_file_to_callback,download_file_to_callback1,
upload_slave_by_buff, upload_slave_by_buff1,
upload_slave_by_sock, upload_slave_by_sock1
and optimize code
Merge branch 'master' of https://github.com/azurewang/lua-resty-fastdfs