PrepareMailQueueManager use an infinite queue

This means if new mails are sent faster than this queue is handled it can crush the memory. That's what is happening on myxwiki.org for example where we get a OOM and a 350MB PrepareMailQueueManager when notifications mails kicks in.

Ideally a BlockingQueue should be used with a limited (and probably configurable) size. That way when the queue is full whatever want to send a new mail wait a bit just the time for the queue to have room.