Add the pending future from ensemble builder to the history

[franchuterivera]

In case there are pending ensemble iterations of ensemble builder when smac is complete, we wait for the ensemble future to finish.

We should add this ensemble iteration to the ensemble history as it can produce a new better performing model.

[codecov]

Codecov Report

Merging #1046 (c6dd8c3) into master (f255a0f) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
- Coverage   85.49%   85.45%   -0.04%     
==========================================
  Files         127      127              
  Lines       10139    10178      +39     
==========================================
+ Hits         8668     8698      +30     
- Misses       1471     1480       +9     

Impacted Files	Coverage Δ
autosklearn/automl.py	84.69% <100.00%> (-0.33%)	⬇️
autosklearn/util/backend.py	74.73% <0.00%> (-1.43%)	⬇️
...ature_preprocessing/select_rates_classification.py	85.91% <0.00%> (ø)
autosklearn/util/logging_.py	88.09% <0.00%> (+0.79%)	⬆️
...ine/components/classification/gradient_boosting.py	92.17% <0.00%> (+0.86%)	⬆️
autosklearn/util/dependencies.py	93.22% <0.00%> (+1.69%)	⬆️

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