feat: Use IMAP APPEND command to upload sync messages (#5845)

Why:
- With IMAP APPEND we can upload messages directly to the DeltaChat folder (for non-chatmail
  accounts).
- We can set the `\Seen` flag immediately so that if the user has other MUA, it doesn't alert about
  a new message if it's just a sync message (there were several such reports on the support
  forum). Though this also isn't useful for chatmail.
- We don't need SMTP envelope and overall remove some overhead on processing sync messages.

src/imap.rs

@@ -32,6 +32,7 @@ use crate::contact::{Contact, ContactId, Modifier, Origin};
 use crate::context::Context;
 use crate::events::EventType;
 use crate::headerdef::{HeaderDef, HeaderDefMap};
+use crate::log::LogExt;
 use crate::login_param::{
     prioritize_server_login_params, ConfiguredLoginParam, ConfiguredServerLoginParam,
 };
@@ -1043,6 +1044,52 @@ impl Session {
         Ok(())
     }
 
+    /// Uploads sync messages from the `imap_send` table with `\Seen` flag set.
+    pub(crate) async fn send_sync_msgs(&mut self, context: &Context, folder: &str) -> Result<()> {
+        context.send_sync_msg().await?;
+        while let Some((id, mime, msg_id, attempts)) = context
+            .sql
+            .query_row_optional(
+                "SELECT id, mime, msg_id, attempts FROM imap_send ORDER BY id LIMIT 1",
+                (),
+                |row| {
+                    let id: i64 = row.get(0)?;
+                    let mime: String = row.get(1)?;
+                    let msg_id: MsgId = row.get(2)?;
+                    let attempts: i64 = row.get(3)?;
+                    Ok((id, mime, msg_id, attempts))
+                },
+            )
+            .await
+            .context("Failed to SELECT from imap_send")?
+        {
+            let res = self
+                .append(folder, Some("(\\Seen)"), None, mime)
+                .await
+                .with_context(|| format!("IMAP APPEND to {folder} failed for {msg_id}"))
+                .log_err(context);
+            if res.is_ok() {
+                msg_id.set_delivered(context).await?;
+            }
+            const MAX_ATTEMPTS: i64 = 2;
+            if res.is_ok() || attempts >= MAX_ATTEMPTS - 1 {
+                context
+                    .sql
+                    .execute("DELETE FROM imap_send WHERE id=?", (id,))
+                    .await
+                    .context("Failed to delete from imap_send")?;
+            } else {
+                context
+                    .sql
+                    .execute("UPDATE imap_send SET attempts=attempts+1 WHERE id=?", (id,))
+                    .await
+                    .context("Failed to update imap_send.attempts")?;
+                res?;
+            }
+        }
+        Ok(())
+    }
+
     /// Stores pending `\Seen` flags for messages in `imap_markseen` table.
     pub(crate) async fn store_seen_flags_on_imap(&mut self, context: &Context) -> Result<()> {
         let rows = context