This adds WP_SESSION_ITEM_FEATURE_ACTIVE and WP_SESSION_ITEM_FEATURE_EXPORTED
features, so _activate and _export APIs have been removed. Modules and unit
tests have also been updated.
+ expose the export transition in the session item class
+ make the export-related flags immutable
+ add an export error flag
+ update and improve documentation
+ replace calling execute_step(..., STEP_ERROR) with rollback
+ implement deactivate internally using rollback
This unifies deactivation steps, which are common between deactivate()
and calling execute_step() with WP_TRANSITION_STEP_ERROR at the
end of a failed activation transition.
