[cleanup] add some notes

[optimisation] _select_best is used very intensivly. Peanuts optimization may count :)

[optimization] traced_selection is used very intensively in debug mode optimizing it is worth the pain. Basically : don't do stuff unnecessary when we don't trace anything.

[optimization] improve massive write performance by optimizing hooks selection profiling on some massive deletion showed up that 2/3 of the time was spent in hooks selection. Those changes make it much more acceptable (through selection is still not negligeable): * use one registry for each event, so we've much less hooks to check when emiting an event as well as no more need for the match_event selector. This required ability to put one appobject into several registries, using a __registries__ class attribute. * check for deprecated .enabled at registry initialization time instead of at selection time A very simple HooksManager class has been reintroduce to choose the right registry on call_hooks. Those optimisations leads to a ~x3 factor of time necessary to delete 16000 entities.

[upgrade] after upgrade, fork command to start instance avoid grave uicfg pb (explicit rules all removed...)

merge

[repo creation] removing existing entities of 'single' cardinality relatino should be considered as 'activeintegrity' hook. Also don't disable that category during repo creation to avoid pb such as two default workflows for one entity types