Use lua-resty-redis instead of the ngx_redis2 module

[portablesoft]

As @agentzh commented here:

lua-resty-redis library is much better than the ngx_redis2 module

https://github.com/openresty/srcache-nginx-module/issues/41#issuecomment-182509912

Lua redis client driver for the ngx_lua based on the cosocket API: https://github.com/openresty/lua-resty-redis

Thanks