title: will use request_pipeline increases performance in resty (lua)
body: <p>In lua resty <a href="https://github.com/pintsized/lua-resty-http" rel="nofollow noreferrer">https://github.com/pintsized/lua-resty-http</a>, I saw we can use request_pipeline for requests. I am wandering whether this will increase the performance. While after reading the source code, I found the request_pipeline method is also implemented with the regular send_request, and a loop is used to send each request one at a time.</p>
  
  <p>Seems it cannot help to improve performance, if it is the case, why bothering to have this method?</p>
  
  <p>Thanks</p>