Send bcc-self messages to all own relays

[link2xt]

This is part of the multi-transport (#7357).

For multi-transport we want to send messages to all or some relays of the contacts. We do not have a way to distribute transports yet other than the From field, but "self" contact is different: we always know our own relay list.

So to make self-sent messages more reliable, and especially sync messages (#7640) we should bcc outgoing messages not only to our primary relay, but to all our relays.