prevent purging when deleting fwd of deleted iaddress

There is an edge-case, when removing the last forward of a deleted
mail_object. E.g. email_user with fwd, first delete email_user,
email_iaddress gets marked as deleted. Then delete fwd and the second
delete triggers a purge. Instead the iaddress must be marked deleted and
kept around to reserve the identifier.
1 job for fixDeletionOfFwd in 2 minutes and 35 seconds (queued for 53 seconds)
Status Name Job ID Coverage
  Simpletest
passed simple_test #18953
container

00:02:35