box: fix crash when on_shutdown trigger is removed

Since on_shutdown triggers are fired asynchronously in their own fibers,
they are not referenced by `event_trigger_iterator`. That's why, if
on_shutdown trigger is deleted while it is running (or is scheduled to
run), it will be instantly removed, and when it is finished, segmentation
fault will happen on its finalizer (`func_adapter_end`). Let's reference
all on_shutdown triggers to prevent such situation - they will be deleted
soon along with the whole event subsystem.

Closes #9275

NO_CHANGELOG=bugfix for unreleased feature
NO_DOC=bugfix