Requiring the caller of lookup() to pass in a `create_fun()` was not
such a good idea in hindsight. It leads to awkward call sites and even
more awkward code when trying to find out whether the lookup was
successfull or not.
Returning an optional gives calling code much more flexibility and makes
the code cleaner.
Do increment the key counter after inserting the first querier into the
cache. Otherwise two queriers with the same key will be inserted and
will fail the test. This problem is exposed by the changes the next
patches make to the querier-cache but will be fixed before to maintain
bisectability of the code.
Fixes: #3529
