add connection connected status for client connection

Hi  ellzey

 when send a request to upstream and failed.  I use  
 if (flags & BEV_EVENT_TIMEOUT) {
}

to judge whether it is timeout. But sometimes we need know more about if the connection is established or timeout during the tranfer data .
 so I add a  field named  "connected"  to the connnection struct to do this 
I test the code using  test_proxy undering examples. It works ok 

Thanks
Jing