Improvements to stats stopping criteria verification

The way that it `set_status!` and `get_status` work right now is too verbose.
You have to pass `max_eval`, `max_iter` and `max_time` every time. Probably these should be stored as part of the stats, or as a separate object - this would have to be internal to stats to avoid allocation.

The `done = stats.status != :unknown` is prone to error. A function check on `stats` would be better.