Move removal of chat message tombstones to a separate function

DELETE operation may be slow compared to UPDATE. It is better to do in
a separate job.

src/chat.rs

@@ -2521,18 +2521,22 @@ pub fn delete_device_expired_messages(context: &Context) -> sql::Result<()> {
              WHERE timestamp < ?",
             params![threshold_timestamp],
         )?;
-
-        // Delete hidden messages that are removed from the server.
-        context.sql.execute(
-            "DELETE FROM msgs \
-             WHERE (chat_id = ? OR hidden) \
-             AND server_uid = 0",
-            params![DC_CHAT_ID_TRASH],
-        )?;
     }
     Ok(())
 }
 
+/// Removes from the database locally deleted messages that also don't
+/// have a server UID.
+pub fn prune_tombstones(context: &Context) -> sql::Result<()> {
+    context.sql.execute(
+        "DELETE FROM msgs \
+         WHERE (chat_id = ? OR hidden) \
+         AND server_uid = 0",
+        params![DC_CHAT_ID_TRASH],
+    )?;
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;