Do not get stuck due to adaptive capping (#749)

* Do not get stuck due to adaptive capping

If the incumbent has all the instance-seed pairs covered
and the challenger is ruled out due to adaptive capping,
we cancel all the runs on the current challenger
to proceed to the next challenger.

* Intensification: Discard capped challenger early

Discard a challenger that is ruled out due to adaptive capping
immediately.
Do not wait for the incumbent to be saturated.
Once a challenger is ruled out due to adaptive capping,
it would always fail the adaptive cap test in the future.

* Remove an unnecessary statement

Simplify the fix to issue #748.
Follows a suggestion by @dengdifan:
https://github.com/automl/SMAC3/pull/749/files#r691892051

* Update the documentation of the challenger discard process

Follow a suggestion by @dengdifan:
#749 (comment)

smac/intensification/intensification.py

@@ -404,6 +404,9 @@ def get_next_run(self,
 
                 # If no more time, stage transition is a must
                 if not is_there_time_due_to_adaptive_cap:
+                    # Since the challenger fails to outperform the incumbent due to adaptive capping,
+                    # we discard all the forthcoming runs.
+                    self.to_run = []
                     self.stage = IntensifierStage.RUN_INCUMBENT
                     self.logger.debug(
                         "Stop challenger itensification due "