Auditable email transactions for good constituent service and legal compliance (eg Spam ACT)

Currently the civicrm_mailing_event_queue only stores the email_id that a message was sent to, and not the email address itself.

This means that if an email address is updated after a mailing is sent, the mailing will appear to have been sent to the new email address even if it was sent to the old one.

The actual e-mail address used should be stored (in addition? to the email_id of that address).