prevent OOM due to evaluation data

[yuki-mt]

What is this PR for?

prevent OOM with large List[EvaluateResultDetail] data

This PR includes

• evaluate method returns generator of EvaluateResultDetail (and lastly return EvaluateResult)
• save EvaluateResultDetail one by one
• load EvaluateResultDetail one by one

What type of PR is it?

Feature/Bugfix/....

What is the issue?

fixed #25

How should this be tested?

python -m unittest

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #44 into master will decrease coverage by 0.45%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   88.06%   87.61%   -0.46%     
==========================================
  Files          15       15              
  Lines         863      872       +9     
==========================================
+ Hits          760      764       +4     
- Misses        103      108       +5

Impacted Files	Coverage Δ
rekcurd/data_servers/__init__.py	96.29% <100%> (+0.17%)	⬆️
rekcurd/core/rekcurd_worker.py	76.47% <100%> (ø)	⬆️
rekcurd/core/rekcurd_dashboard_servicer.py	93.04% <70.58%> (-4.26%)	⬇️

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[keigohtr]

Almost LGTM. Great work with you!
I made some questions, could you give me a feedback?

[keigohtr]

LGTM