Add support for tcp_user_timeout

[drdrsh]

TCP_USER_TIMEOUT is a quite powerful tool for managing TCP timeouts. It is a bit complex in how it behaves but it is currently the only real way for controlling the number of retransmits an active socket makes before giving up and timing out. Without this parameter set on the socket, we fallback to the OS default tcp_retries2 parameter which generally keeps retrying for about 15 minutes before declaring the connection dead.

using TCP_USER_TIMEOUT with keepalive parameter ensures that we detect bad connections early and clean them up before we exhaust our pools and lock up the proxy.

Docs:
https://man7.org/linux/man-pages/man7/tcp.7.html
https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/
https://tech.instacart.com/the-vanishing-thread-and-postgresql-tcp-connection-parameters-93afc0e1208c

[levkk]

It would be good to make it optional (leave default as not configured), but can be done as follow up if deemed necessary.