And fix a bug in the handling of ephemeron during minor collection
found thanks to this test. Oldifying of ephemerons' data was not iterated
enough because `oldify_todo_list` doesn't become non-null each time
`caml_oldify_one` is run.
During clean phase for consistency we need to check if the key
checked or got is alive or not. Three possibilities:
- Doesn't unset the key, but still return the right value as if it
- Unset the key and the data
- Clean the ephemeron
Testing aliveness is a little costly, so it is better to amortize
this cost. Previously the last possibility was implemented but a
better trade-off seems the second one, since the clean phase is still
going to clean the ephemeron eventually.