Do not load all nodes/comments with loadMultiple()

Problem/Motivation

Notify loads all nodes/comments in database using

• $this->entityTypeManager->getStorage('node')->loadMultiple()
• $this->entityTypeManager->getStorage('comment')->loadMultiple();

in \Drupal\notify\Notify::send().

When the site has many nodes (like 50,000+ nodes), PHP uses up memory / execution time with those lines.

Proposed resolution

Do not use loadMultiple(), use entityQuery or SelectQuery instead and gather entity-id / created / changed used later.

Attached is the patch.