Also, send the message text in the post-message too, we can't support the workaround for duplicated
message text shown in ancient versions anymore, otherwise newer versions won't show the message text
at all.
On the receiving side, download post messages w/o known pre-message after 30s to protect from lost
messages.
This isn't easy to test in Rust though because the `smtp` sending code requires an SMTP connection,
so it's only tested (by already existing tests) that messages are queued in the right order.
Another problem is that the `smtp` sending logic doesn't try to send any messages following a
failed-to-send message until the retry limit is reached, so if there's smth wrong with a
post-message, other unrelated messages are delayed, but this problem has already existed.
Iroh-Gossip-Topic is sent in a post-message. Post-message goes to trash,
so topic should be associated with the existing pre-message that is
updated rather than with the post-message.
