Implement debug mode with debug logging level

[quantum5]

This avoids spam but allows it when explicitly asked for.

[Xyene]

LGTM

[codecov-commenter]

Codecov Report

Merging #989 (36c24a1) into master (f5cf158) will increase coverage by 2.97%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #989      +/-   ##
==========================================
+ Coverage   80.82%   83.79%   +2.97%     
==========================================
  Files         139      139              
  Lines        4714     4716       +2     
==========================================
+ Hits         3810     3952     +142     
+ Misses        904      764     -140     

Impacted Files	Coverage Δ
dmoj/judge.py	52.83% <ø> (+1.25%)	⬆️
dmoj/judgeenv.py	56.84% <100.00%> (+0.92%)	⬆️
dmoj/executors/java_executor.py	85.35% <0.00%> (+2.02%)	⬆️
dmoj/executors/compiled_executor.py	77.71% <0.00%> (+4.00%)	⬆️
dmoj/cptbox/tracer.py	77.33% <0.00%> (+18.34%)	⬆️
dmoj/cptbox/handlers.py	100.00% <0.00%> (+25.00%)	⬆️
dmoj/cptbox/isolate.py	86.77% <0.00%> (+47.93%)	⬆️
dmoj/control.py	100.00% <0.00%> (+64.70%)	⬆️

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