Fix initialization race (#12374)

* block all the sync startup code on init signal

* don't need chainStarted if everything blocks

* set empty clock by default to work around panics

* remove unused clock, zero-value for init-sync

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Co-authored-by: Kasey Kirkham <kasey@users.noreply.github.com>
Co-authored-by: Nishant Das <nishdas93@gmail.com>

beacon-chain/sync/initial-sync/service.go

@@ -67,6 +67,7 @@ func NewService(ctx context.Context, cfg *Config) *Service {
 		chainStarted: abool.New(),
 		counter:      ratecounter.NewRateCounter(counterSeconds * time.Second),
 		genesisChan:  make(chan time.Time),
+		clock:        startup.NewClock(time.Unix(0, 0), [32]byte{}), // default clock to prevent panic
 	}
 
 	return s