Add ability to resend emails synchronously

Imagine that you have millions of emails to resend. If you try to resend them asynchronously then the prepare and send queues will reach their full capacity and will refuse more emails, leading to an exception. In addition if you increase the queue sizes to a large value then you'll hit a memory issue since the queues are in memory.

Thus it's interesting to be able to resend emails one by one in this case.

Note that a future improvement will be to be able to resend by batches, ie. batches of max queue sizes, but there might still be use cases for resending synchronously one by one if you wish to keep a low memory usage and if your code is executed in a separate thread.