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).

Issue #2144263 by fago, yched, alexpott, Berdir, plach, andypost, effulgentsia, michaelfavia: Decouple entity field storage from configurable fields.

Issue #2190313 by pfrenssen, Berdir, amateescu, ianthomas_uk: Add $EntityType::load() and loadMultiple() to simplify loading entities.

Issue #1709960 by Berdir, zserno, jessebeach | joachim: Declare a maximum length for entity and bundle machine names.