Right now, when a new ovs container image is rolled, all OVS pods will restart; on new vswitchd process start, it will flush all kernel flows. This will disrupt gateway datapath, until ovn-controller reconnects to vswitchd and reinstalls its flows again.

To avoid it, we can adopt flow-restore-wait option for vswitchd. It is implemented as `reload` action on vswitchd systemd service unit. Since we don't use systemd units in podified env, we need to reimplement it in our service stop/startup scripts.

In PreStop:

• dump flows to a file on PVC;

In PreStart:

• set flow-restore-wait=true;
• start vswitchd;
• once vswitchd is up:
  • restore flows;
  • set flow-restore-wait=false to allow ovn-controller to reconnect.

When backing flows up, consider how long it may take, and if the default pod timeouts for startup / liveness are long enough to never hit a forced kill before vswitchd is started. (alternatively, consider modifying liveness checks to monitor flow dump progress.)