Issue #2337927 by effulgentsia, plach, fago: Fixed SqlContentEntityStorage::onFieldStorageDefinition(Create|Update|Delete)() is broken for base fields.

Issue #2333113 by effulgentsia, plach: Add an EntityDefinitionUpdateManager so that entity handlers can respond (e.g., by updating db schema) to code updates in a controlled way (e.g., from update.php).