title: in app purchase, SKPaymentTransactionStatePurchased retries
body: <p>I'm implementing In App Purchase and am validating receipts on my server.  </p>
  
  <p>When I encounter <code>SKPaymentTransactionStatePurchased</code> during <code>paymentQueue:updatedTransactions:</code> I send the receipt to my server for validation.  When my server responds with an OK I call <code>finishTransaction</code>.</p>
  
  <p>If my server is offline or I'm otherwise unable to validate the receipt, what triggers the app to call <code>paymentQueue:updatedTransactions:</code> again, effectively giving me a chance to retry the validation?  I've noted that restarting the app does this - is there anything else?  Will it get called again on some fixed frequency while the app remains running?</p>
  
  <p>Is there a manual way to force the transaction-queue to cycle?</p>