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 is not good enough. How do we know which emails failed to be sent? In addition you're sending an exception only for the frist error... what about all the rest?