Simplify CiviMail-generated addresses and add the optional localpart+ prefix

As we dropped domain_id and don't really need the VERP-encoded email address (as we can get that through contact_id), it's high time to simplify email addresses used by CiviMail.

1. Drop domain_id from the email addresses that carry it.
2. Drop VERP-encoded contact emails to make the addresses shorter.
3. Add the possibility of using localpart+ prefix.
4. Update the CiviMail Preprocessor to cater for these new address formats (while still working with the old addresses).
5. Update AMaViS as above.