support tcp_keepalive

[jiemingwang]

Hi team,
Can we support tcp keepalive for server socket ? I understand that uvicorn provides a simiplar timer at http layer (timeout_keep_alive). But there is a problem when there is a network device (firewall) between client and server. The device can monitor the connection and drop it when idle time exceeds the timer set by network device (firewall_timeout), but is still less than timeout_keep_alive.
E.g.,
firewall_timeout=60
timeout_keep_alive=90

So in this case, a connection will be dropped when idle time exceeds 60 seconds.

By enabling tcp_keepalive, a connection will treated as active and prevent from being dropped (i.e., by firewall) as long as tcp keepalive packet is sent within firewall_timout, even though there is no http data sent.

I have tweaked the uvicorn codes to support tcp_keepalive. It should be a small change. But just want to consult with team here before I raise an issue ?

Thanks
J. Wang