All messages to be sent need to be visible in the Mail Status UI before they start to be sent

This is also required to prevent having only N message sent, then have XWiki crashing and loose all the other messages not sent yet.

Current Algorithm:

• Put message reference on the sending queue
• Sender queue creates the message, it's serialized to disk and then sent

What we need instead:

• Put message reference on a prepare queue
• Have a Mail Prepare Thread take all message from that prepare queue, create the message one by one and serialize them one by one and put them on a sending queue
• Have the Mail Sender Thread read from the sending queue, load the message from disk and send it