server: should signal readiness (--background) later

[tbg]

See #34897 (comment)

TL;DR: --background currently signals readiness here:

cockroach/pkg/server/server.go

Lines 1375 to 1380 in 475958d

	if len(bootstrappedEngines) > 0 {
	// The cluster was already initialized.
	doBootstrap = false
	if s.cfg.ReadyFn != nil {
	s.cfg.ReadyFn(false /*waitForInit*/)
	}

But that's too early - it may still take a while (tens of seconds) until the node is actually responsive.

Jira issue: CRDB-4609

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