jetstream liveness check interval is too short and it should be configurable.

[itaru2622]

as described in bluesky-social/atproto#2901,

When I tested jetstream in selfhosting env, It restarts every 15 seconds.

it caused by below part:

jetstream/cmd/jetstream/main.go

Lines 174 to 192 in 39f24fb

	// Start a goroutine to manage the liveness checker, shutting down if no events are received for 15 seconds
	shutdownLivenessChecker := make(chan struct{})
	livenessCheckerShutdown := make(chan struct{})
	go func() {
	ticker := time.NewTicker(15 * time.Second)
	lastSeq := int64(0)
	log := log.With("source", "liveness_checker")
	for {
	select {
	case <-shutdownLivenessChecker:
	log.Info("shutting down liveness checker")
	close(livenessCheckerShutdown)
	return
	case <-ticker.C:
	seq, _ := c.Progress.Get()
	if seq == lastSeq && seq != 0 {
	log.Error("no new events in last 15 seconds, shutting down for docker to restart me", "seq", seq)
	close(livenessKill)

15 seconds may be suitable when it serving whole world with lots of users, but not for other cases.
so it should be configurable.