fix: move sorting outside of SQL query in store_seen_flags_on_imap

With `ORDER BY` statement SQLite searches
the `imap` table by `transport_id` and for each found row
scans the whole `imap_markseen` table.
Number of `imap` entries for each `transport_id`
is usually large as we need to know
which UIDs to delete on IMAP server
when deleting a message.

```
sqlite> EXPLAIN QUERY PLAN
SELECT imap.id, uid, folder FROM imap, imap_markseen
WHERE imap.id = imap_markseen.id
AND imap.transport_id=?
AND target = folder
ORDER BY folder, uid;
QUERY PLAN
|--SEARCH imap USING INDEX sqlite_autoindex_imap_1 (transport_id=?)
`--SCAN imap_markseen
```

Without `ORDER BY` statement SQLite scans `imap_markseen`
table which is expected to be small,
and then searches `imap` table by `rowid` for each found result.

```
sqlite> EXPLAIN QUERY PLAN
SELECT imap.id, uid, folder FROM imap, imap_markseen
WHERE imap.id = imap_markseen.id
AND imap.transport_id=?
AND target = folder;
QUERY PLAN
|--SCAN imap_markseen
`--SEARCH imap USING INTEGER PRIMARY KEY (rowid=?)
```

Query planning was tested with SQLite 3.52.0.
It is possible to explictly make
query planner move sorting to the last step
with `ORDER +folder, +uid`, but this is not recommended
in SQLite documentation
(see <https://www.sqlite.org/optoverview.html#uplus>).

It is also possible to add indexes,
but indexes use space,
adding them requires an SQL migration,
and each index needs to be updated so it will slow down writes.

src/imap.rs

@@ -1104,14 +1104,13 @@ impl Session {
         }
 
         let transport_id = self.transport_id();
-        let rows = context
+        let mut rows = context
             .sql
             .query_map_vec(
                 "SELECT imap.id, uid, folder FROM imap, imap_markseen
                  WHERE imap.id = imap_markseen.id
                  AND imap.transport_id=?
-                 AND target = folder
-                 ORDER BY folder, uid",
+                 AND target = folder",
                 (transport_id,),
                 |row| {
                     let rowid: i64 = row.get(0)?;
@@ -1122,6 +1121,16 @@ impl Session {
             )
             .await?;
 
+        // Number of SQL results is expected to be low as
+        // we usually don't have many messages to mark on IMAP at once.
+        // We are sorting outside of SQL to avoid SQLite constructing a query plan
+        // that scans the whole `imap` table. Scanning `imap_markseen` is fine
+        // as it should not have many items.
+        // If you change the SQL query, test it with `EXPLAIN QUERY PLAN`.
+        rows.sort_unstable_by(|(_rowid1, uid1, folder1), (_rowid2, uid2, folder2)| {
+            (folder1, uid1).cmp(&(folder2, uid2))
+        });
+
         for (folder, rowid_set, uid_set) in UidGrouper::from(rows) {
             let folder_exists = match self.select_with_uidvalidity(context, &folder).await {
                 Err(err) => {