limit rate of webxdc updates (#3417)

* more flexible render_webxdc_status_update_object()

* delay webxdc updates when ratelimit is reached

* inject updates messages to the SMTP loop as needed

this avoids starting several tasks
and allows sending updates out after a restart of the app.

* use mutex to prevent race conditions in status updates

* check ratelimiter only before the sending loop; it won't change until messages are actually sent out

* fix typo

* prefer standard type declaration over turbofish syntax

* use UNIQUE and ON CONFLICT for query smtp_status_updates

* combine DELETE+SELECT to one atomic statement

* as all operations on smtp_status_updates are now atomic, a mutex is no longer needed

* test DELETE+RETURNING statement

* simplify calls to can_send()

* comment about ratelimit boolean in send_smtp_messages()

src/sql/migrations.rs

@@ -637,6 +637,19 @@ CREATE INDEX smtp_messageid ON imap(rfc724_mid);
         )
         .await?;
     }
+    if dbversion < 91 {
+        info!(context, "[migration] v91");
+        sql.execute_migration(
+            r#"CREATE TABLE smtp_status_updates (
+              msg_id INTEGER NOT NULL UNIQUE, -- msg_id of the webxdc instance with pending updates
+              first_serial INTEGER NOT NULL, -- id in msgs_status_updates
+              last_serial INTEGER NOT NULL, -- id in msgs_status_updates
+              descr TEXT NOT NULL -- text to send along with the updates
+            );"#,
+            91,
+        )
+        .await?;
+    }
 
     Ok((
         recalc_fingerprints,