XWIKI-11486: Batch API to send large volumes of mail

* Create xwiki-platform-mail-send-storage module structure
* Create iterators (GroupMimeMessageIterator, UsersMimeMessageIterator, SerializedFilesMimeMessageIterator)
* Add factories for different iterators
* Rename MailResultListener and create Component
* Change returned type of MailSender#send and MailSender#sendAsynchronously
This doesn't look good since it means having a single javamail session for the lifetime of XWiki. Doesnt sound right.... For example imagine that the xwiki users changes the configruation through the Admin UI... the Session won't change and will thus have wrong values...