jsonrpc: Remove message id from chatlist item (#3071)

Why? because desktop currently fetches the chatlist multiple times, even though it just needs the
chatlistitem for one chat.

Note: @r10s was worried before that exposing the method to get a single updated chatlistitem could
lead to race conditions where the chatlist item is newer than the chatlist order. But I don't think
this will change anything for desktop besides making it a little faster (because currently desktop
fetches the whole chatlist instead of just the entry it needs when an entry updates).

src/chatlist.rs

@@ -362,6 +362,25 @@ pub async fn get_archived_cnt(context: &Context) -> Result<usize> {
     Ok(count)
 }
 
+/// Gets the last message of a chat, the message that would also be displayed in the ChatList
+/// Used for passing to `deltachat::chatlist::Chatlist::get_summary2`
+pub async fn get_last_message_for_chat(
+    context: &Context,
+    chat_id: ChatId,
+) -> Result<Option<MsgId>> {
+    context
+        .sql
+        .query_get_value(
+            "SELECT id
+                FROM msgs
+                WHERE chat_id=?2
+                AND (hidden=0 OR state=?1)
+                ORDER BY timestamp DESC, id DESC LIMIT 1",
+            (MessageState::OutDraft, chat_id),
+        )
+        .await
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;