Possible race condition when comparing legacy and new store size in the migration step of the DW

The DW contains a step to migrate events stored in the legacy events store in the to the new one. It's enabled automatically when there is more legacy events than new events but since event stores are asynchronous it's actually not always so easy to compare them when they actually have the same number of events and there is some storage triggered at the XWiki init (which are generally bugs to fix but still would be nice to have a less strict step check).