fix(iroh-gossip): do not drop existing peer connection when we get incoming one

[link2xt]

Otherwise if two peers try to connect to each other at the same time, they may end up creating two connection and then closing both of them. Better keep two one-directional connections.

deltachat-core-rust will have a downstream test that reliably triggers the problem at deltachat/deltachat-core-rust#5595
I did not manage to reproduce the problem in Rust, it seems to depend on timing and is triggered in the test which sends tickets around using SMTP.

Fixes #2307

Replaces #2308

[divagant-martian]

this can be done with send_rx.is_closed()

[link2xt]

I have force-pushed already, but is_closed() looks better IMO.

[Frando]

This looks sensible to me!

When rereading the connection handling in iroh_gossip::net, I think this could need some more love, which I will try to have a look at soon. We really should have only a single connection between two peers, and disambiguate by node id order like we do in iroh-sync. Currently I think it can happen, depending on timings, that there are actually two connections between peer A and peer B (one from A dialing B, and one from B dialing A) which does not matter because they are handled in the same way but still is unfortunate. However this is independent of this concrete issue, so I am fine with merging this if it does fix the issue deltachat has. And I will look into a better way to structure the connection code independently.

[link2xt]

This also should be merged to the main besides maint-0.16, but for delta chat would be nice to have it at least as a 0.16 bugfix release, then there is a chance we get (disabled-by-default) realtime channels in the next desktop release.

[dignifiedquire]

maint-0.16 exist because there were issues when cutting the release, given that we are cutting a release every 2-3 weeks currently I would much more prefer to merge this into main and DC just upgrading to that commit. And you can switch over to the released version in a week.

The breaking changes on main are mostly renames which are easy to adapt to.

[link2xt]

Replaced with #2318 which is based on main