Setting new added relay as default fails to sync

[nicodh]

It seems that adding a new relay and immediately setting that relay as default fails to sync.

I tried with 2 desktop instances (2.35.0), an Android (2.34.0) & iOS (2.33.0) client, running as multiple devices.

So the reproducible path is:

1. create a new relay on device1 - the other clients show the new relay
2. set the new relay as default on device1 - the other clients won't get that update
3. switch to another (already existing relay) on device1 - the clients get updated
4. switch to the new relay on another device (that received the new relay) - all clients get updated

Seems to be a core issue since it doesn't matter if you use Android or Desktop as device1.

[iequidoo]

I'm adding a test for this in #7638 and it indeed fails

[link2xt]

The problem is that when you add a new relay, switch to it as a primary one and start sending sync messages there, the other device may have not configured this new relay yet. When it receives info about this new relay and connects to it, it will not download sync messages already sent there.

I think the solution to this is:

1. Revert feat: Use IMAP APPEND command to upload sync messages (#5845) #5901 and switch back to sending sync messages over SMTP. This was added to support "shared mailbox" usecase and was useful because uploading over IMAP to mvbox folder does not trigger notifications and uses IMAP COMPRESS, but now shared mailbox is not supported and this conflicts with multi-relay as uploading sync messages multiple times with compression is worse than sending once to all your relays without compression. Besides that, there are reordering problems. (created Stop using IMAP APPEND to upload sync messages #7651)
2. Send sync messages to all relays, not just the primary one. This way sync message with new From address will be received on the old relay as well and we know that this is the new primary relay. Still no need for explicit "primary relay" sync message.

[iequidoo]

The solution looks good, but we also need to restart I/O or at least start it for the new relay, this is also a problem which i saw when debugging primary relay sync.

[link2xt]

Next step for this is #7651