review/enhance behavioural consistency of the id generation process

[RonnyPfannschmidt]

in #4740 we noted that there was a slight behavioral inconsistency in how we generate ids and how values are passed around in between real ids, the idfn, the id generation hook,

we kept the original behavior, but we really need a corrected outline on the behavior

im proposing this for pytest 6.0 as the correction may have the unwanted consequence of changing some test ids due to now passing trough the hook always

another outcome may be to iron out the cases where the call order should be as it is, and specifying it + tightening that contract with unit-tests

[nicoddemus]

@RonnyPfannschmidt you still want to tackle this for 6.0?

[nicoddemus]

@RonnyPfannschmidt gentle ping. Do you think you will be able to tackle this soon?

[RonnyPfannschmidt]

Unfortunately not within the next month

[nicoddemus]

OK no problem, thanks. I've removed it from the 6.1 milestone then, I don't think it is urgent for that release. Feel free to reschedule it to another version if you would like.