add  TCP SO_KEEPALIVE support for HTTP upstream.

Add TCP SO_KEEPALIVE support for HTTP upstream.

test conf: 
```
......
upstream backend {
    server  www.baidu.com;

    keepalive 16; #optional.
    so_keepalive tcp_keepidle=3 tcp_keepintvl=3 tcp_keepcnt=3;
}
...........
location ^~/index.html {
    proxy_pass http://backend/;
    proxy_http_version 1.1;
    proxy_set_header Connection "";
    proxy_set_header Host www.baidu.com;
}
```
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