We have ended up not using them, so let's not carry them
in the ABI of 0.4
We can always revert that, but let's first decide how
these objects should be used
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.
