max_eval_time_mins parameter doesn't stop a long-running eval

During `fit()`, I noticed that some evaluations were running for over an hour even though the `max_eval_time_mins=5`.
This is because the code is not actually stopping the thread doing the eval.

In `class Interruptable_cross_val_score`, `stop()` is not actually interrupting the thread, but instead calling `self._stopevent.set()`, which doesn't stop the thread because nothing is checking that event, and then waiting for the thread to stop on its own.

Python doesn't have a good way to interrupt threads. See the discussion at https://stackoverflow.com/questions/323972/is-there-any-way-to-kill-a-thread-in-python

Given that `_wrapped_cross_val_score()` is already running in a separate process due to joblib, one solution would be to make `Interruptable_cross_val_score` a daemon thread, and then remove the call to `tmp_it.stop()` in `_wrapped_cross_val_score()`. Thus when the timeout passes, the process will exit cleanly.