Refactor usage of Display class into Oracle

[haifeng-jin]

To avoid the Display to call the chief from a client after calling end_trial() on the last trial, which would requie the chief to wait even all the trials were reported to be finished, we refactor the Display class into the Oracle.
Now, no client would print logs, but only the chief would do.

Another important change is that the chief would start to block when calling search(). It used to block in __init__().

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (354b2cb) 97.60% compared to head (ee81678) 97.61%.

Additional details and impacted files

@@             Coverage Diff             @@
##           timeout     #959      +/-   ##
===========================================
+ Coverage    97.60%   97.61%   +0.01%     
===========================================
  Files           50       50              
  Lines         3086     3107      +21     
===========================================
+ Hits          3012     3033      +21     
  Misses          74       74              

Files	Coverage Δ
keras_tuner/engine/base_tuner.py	100.00% <100.00%> (ø)
keras_tuner/engine/oracle.py	100.00% <100.00%> (ø)
keras_tuner/engine/tuner_utils.py	100.00% <ø> (ø)

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