The mail sender module store a lot of useless data in the prepare queue

When sending a mail asynchronous the ExecutionContext is cloned and stored in a queue with the mail message. Problem is that this cloned context might contain quite a lot of totally useless stuff to remember.

For example in a myxwiki.org dump I can see tons of PrepareMailQueueItem objects each retaining about 65kB of RAM mostly coming from the ScriptContex. When a lot of mail are sent very quickly (like when sending notification mails) it can quickly explode the RAM combined with XWIKI-16309.