TS-1710: esi plugin enhancement such as support forward proxy

  * bug fixed: do NOT support keep-alive for async requests,
    ignore header Proxy-Connection
  * save header for fetch event when http status is not 200
  * getRequestStatus judge http status
  * node type change from uint32_t to enum for changing from "if ...
    else if" to "switch"
  * esi:include tag allows empty url

Signed-off-by: Yu Qing <zhuangyuan@taobao.com>
Signed-off-by: Zhao Yongming <ming.zym@gmail.com>