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 class could be implemented as an internal perlookup component in order to support injection, and have some set method (not part of the interface!) to setup the needed parameters. Since the user will go through the factory they won't see it and since it's internal they're not supposed to use it. I'm not sure which is best.