CiviMail cron needs to be able to run from the originating domain

This patch allows CiviMail to send by originating domain for multisites.

It fixes 2 issues we have been hitting:

1) Multisites users have permissions according to the domain they log onto. CiviMail cron respects (sort-of - see 7223) those permissions but it doesn't know which site the mail was sent from and the user may have different permissions on different sites. This patch sets the domain id when it is written & only that domain picks up the mail

2) if the e-mails are only sent by the originator domain the URL links are all to the originator domain

Tested & working