Issue #1824024 by mikeryan: Destination and field handlers may now be registered through hook_migrate_api(). Automatic registration of all migration and handler classes may be disabled at admin/content/migrate/registration. The handler configuration formerly at admin/content/migrate/configure is now at admin/content/migrate/handlers.

Issue #1778952 by mikeryan: Add explicit auto-registration of non-dynamic migrations, remove performing registration on cache clear