Adding a socket handler for keep TCP connection (#3422)

This is not http keepalive option it is TCP level
which is lower than HTTP.

It will send meanless ack flag packet to server.
so it woudn't be drop TCP connection by server side
like Firewall, Loadbalancer, Nginx etc.

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/adapters/http.js

@@ -313,6 +313,12 @@ module.exports = function httpAdapter(config) {
       reject(enhanceError(err, config, null, req));
     });
 
+    // set tcp keep alive to prevent drop connection by peer
+    req.on('socket', function handleRequestSocket(socket) {
+      // default interval of sending ack packet is 1 minute
+      socket.setKeepAlive(true, 1000 * 60);
+    });
+
     // Handle request timeout
     if (config.timeout) {
       // This is forcing a int timeout to avoid problems if the `req` interface doesn't handle other types.