add ngx_http_reqstat_module
Merge pull request #446 from cfsego/reqstat_new

可以基于虚拟主机统计的统计模块
add doc for ngx_http_reqstat_module
Merge pull request #451 from cfsego/new_reqstat

add doc for ngx_http_reqstat_module
bugfix 回滚
Merge pull request #454 from dinic/session_sticky_boke_bugfix

it looks good to me
Merge commit 'alibaba/master' into tengine_2.0.3
updated change log according to zyk's suggestion