Only discard success mail statuses when the preparation is over

Right now the mail.sender.database.discardSuccessStatuses config property imply that success statuses are never saved in the DB.

We should change that by:

• saving them in the Db during the prepare phase
• discard them only if the prepare phase has finished successfully

This is required to implement XWIKI-11652 since we need to exclude all recipients who have successfully received the mail whenever there's an error in the prepare phase.