Make smtp-queue batch resends by destination

[Ekleog]

Idea: we can batch resends by destination, if the other server is down then we can just delay the whole destination across mails, if the mailbox is down then we can delay all mails to this mailbox/destination, etc.

This still require some more thought about how to make this work in a way that still allows the network destination to not be the same as the RCPT TO destination (eg. filters or relays)