Revert to default max_trials inf and pool-size 1

[bouthilx]

The new default behavior is confusing for users. It is also difficult to determine a good default max_trials, so having not enough or to many trials sampled by default at the start of HPO can be annoying for many users. Using inf by default and iterating with pool-size may be the best alternative.

Now that we have a support for n-workers, the argument pool-size we previously deprecated could actually make sense. By default, pool-size should be equal to number of workers. We have n-workers set to 1 by default, so by default we would be back to previous behavior; sampling 1 trial at a time, until max_trials.

@mgermain