remove keepalive (since go 1.13 default is 15s)

https://www.reddit.com/r/golang/comments/d7v7dn/psa_go_113_introduces_15_sec_server_tcp/

pgconn/config.go

@@ -800,7 +800,7 @@ func parsePort(s string) (uint16, error) {
 }
 
 func makeDefaultDialer() *net.Dialer {
-	return &net.Dialer{KeepAlive: 5 * time.Minute}
+	return &net.Dialer{}
 }
 
 func makeDefaultResolver() *net.Resolver {