Reduce number of synchronizations GODT-313

* [x] expononential cooldown of retries
* [x] do not trigger sync by counts
* [x] randomization of event poll interval

internal/store/store.go

@@ -104,6 +104,7 @@ type Store struct {
 	imapUpdates chan interface{}
 
 	isSyncRunning bool
+	syncCooldown  cooldown
 	addressMode   addressMode
 }
 
@@ -148,6 +149,9 @@ func New(
 		log:          l,
 	}
 
+	// Minimal increase is event pollInterval, doubles every failed retry up to 5 minutes.
+	store.syncCooldown.setExponentialWait(pollInterval, 2, 5*time.Minute)
+
 	if err = store.init(firstInit); err != nil {
 		l.WithError(err).Error("Could not initialise store, attempting to close")
 		if storeCloseErr := store.Close(); storeCloseErr != nil {