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 comment doesn't help: JavaMail already has a Session object that can be instantiated, why do we need a component for that?

Generally speaking a good javadoc comment is never about "what" but about "why"!