fix: Mark a message as delivered only after it has been fully sent out (#8062)

Fix https://github.com/chatmail/core/issues/8042

The problem was that after receiving the bcc_self'ed pre-message in
`receive_imf`, the logic there only looked for a pending
`smtp`-table-entry that matches the rfc724_mid, and if there was none
then it thought "Great, apparently the message is fully sent out, we can
mark it as delivered!".

But with pre-messages, the same message can have two `smtp` entries (one
for the pre-message and one for the post-message), and the message
should only be marked as delivered once both of them are sent out.

Now, I changed the logic to look for all entries with the same msg_id.
This is actually the same SQL query used in smtp.rs, so, I extracted it
into a new function; feel free to suggest a better name for it.

I tested on Android that it now works fine.

I'll add a test in a follow-up PR.

There are a lot of other problems with sending large files, though:
- The pre-message is sent before the post-message, so that for the
receiver it looks as if the message arrived, but stays in
"downloading..." forever
- There is quite a time delay between clicking on "Send" and the
outgoing message appearing in the chat
- The message shortly gets a letter icon right after it is sent
- I'm wondering if there is a way to give feedback to the user
immediately if the message is too big
- It's unclear when exactly we want to send read receipts

I'll open a follow-up issue for these.

src/smtp.rs

@@ -465,11 +465,7 @@ pub(crate) async fn send_msg_to_smtp(
     match status {
         SendResult::Retry => Err(format_err!("Retry")),
         SendResult::Success => {
-            if !context
-                .sql
-                .exists("SELECT COUNT(*) FROM smtp WHERE msg_id=?", (msg_id,))
-                .await?
-            {
+            if !msg_has_pending_smtp_job(context, msg_id).await? {
                 msg_id.set_delivered(context).await?;
             }
             Ok(())
@@ -478,6 +474,16 @@ pub(crate) async fn send_msg_to_smtp(
     }
 }
 
+pub(crate) async fn msg_has_pending_smtp_job(
+    context: &Context,
+    msg_id: MsgId,
+) -> Result<bool, Error> {
+    context
+        .sql
+        .exists("SELECT COUNT(*) FROM smtp WHERE msg_id=?", (msg_id,))
+        .await
+}
+
 /// Attempts to send queued MDNs.
 async fn send_mdns(context: &Context, connection: &mut Smtp) -> Result<()> {
     loop {