fix: lock the database when INSERTing a webxdc update

`query_row_optional` does not hold the write lock
and may fail with "database is locked" error
or cause the other task such as SMTP loop to fail.

src/webxdc.rs

@@ -409,6 +409,7 @@ impl Context {
         instance_id: &MsgId,
         status_update_item: &StatusUpdateItem,
     ) -> Result<Option<StatusUpdateSerial>> {
+        let _lock = self.sql.write_lock().await;
         let uid = status_update_item.uid.as_deref();
         let Some(rowid) = self
             .sql