fixed memory leak while iterating results

[stlrnz]

I've tried to fix the issue described in #8410

[beberlei]

@simPod can you say something?

[stlrnz]

I've checked why the test https://github.com/doctrine/orm/pull/8412/checks?check_run_id=1654953910 failed.

It seems that the ObjectHydrator in contrast to the SimpleObjectHydrator does need $result to be persistent.
So maybe this is not the correct solution for the linked issue?

[simPod]

@stirnz same test run fails for iterate() as well, it was not tested for that case though while toIterable() is. Looking into it.

[simPod]

I remember being tasked with fixing #3238 which introduced the memory leak as we're keeping result in memory now. Looking for workaround.

[beberlei]

Related #8414