refactor(smtp): introduce smtp::Connection structure

[link2xt]

It is now impossible for SMTP client to be connected without having a From address and transport_id available.

Follow-up to 7073049

[Hocuri]

Nice refactoring, good to clean up some old code

[Hocuri]

Nit: I find log_err easier to read than if let Err … warn!, since if let Err … warn! 'hides' the most important thing, namely the call to record_success.

Suggested change

	let transport_id = connection.transport_id;
	if let Err(err) = record_success(context, transport_id).await {
	warn!(
	context,
	"Failed to record successful use of transport {transport_id} in smtp_success table: {err:#}."
	);
	}
	record_success(context, connection.transport_id)
	.await
	.with_context(|| {
	format!("Failed to record successful use of transport {transport_id}")
	})
	.log_err(context)
	.ok();

[Hocuri]

This needs to use the alternate formatting now, otherwise the .context("Envelope error")? will swallow the inner error

Suggested change

	smtp.last_send_error = send_result.as_ref().err().map(|e| e.to_string());
	smtp.last_send_error = send_result.as_ref().err().map(|e| format!("{e:#}"));

[Hocuri]

...or maybe this is where we need to use the alternate formatting? Anyways, can't hurt to use it in both places.

pub enum Error {
    #[error("Send error: {:#}", _0)]
    SmtpSend(async_smtp::error::Error),
    #[error("{:#}", _0)]
    Other(#[from] anyhow::Error),
}

(Github doesn't support to post this as a suggestion, since there are deleted lines in the diff)