refactor: YataiService Store Abstraction

[jackyzha0]

Description

First part of #1540, does the base abstraction for existing YataiService stores so that all session creation is handled by the YataiService implementation rather than within the stores itself. This way, we can add the lock acquisition straight into yatai_service_impl.py.

New module structure:

How Has This Been Tested?

• local unit/integration tests

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature and improvements (non-breaking change which adds/improves functionality)
• Bug fix (non-breaking change which fixes an issue)
• Code Refactoring (internal change which is not user facing)
• Documentation
• Test, CI, or build

Component(s) if applicable

• BentoService (service definition, dependency management, API input/output adapters)
• Model Artifact (model serialization, multi-framework support)
• Model Server (mico-batching, dockerisation, logging, OpenAPI, instruments)
• YataiService gRPC server (model registry, cloud deployment automation)
• YataiService web server (nodejs HTTP server and web UI)
• Internal (BentoML's own configuration, logging, utility, exception handling)
• BentoML CLI

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change reduces project test coverage and requires unit tests to be added
• I have added unit tests covering my code change
• My change requires a change to the documentation
• I have updated the documentation accordingly

[pep8speaks]

Hello @jackyzha0, Thanks for updating this PR.

There are currently no PEP 8 issues detected in this PR. Cheers! 🍻

Comment last updated at 2021-03-26 22:20:11 UTC

[codecov]

Codecov Report

Merging #1541 (e44c028) into master (ce9bdd4) will increase coverage by 0.08%.
The diff coverage is 68.97%.

@@            Coverage Diff             @@
##           master    #1541      +/-   ##
==========================================
+ Coverage   68.85%   68.94%   +0.08%     
==========================================
  Files         152      153       +1     
  Lines       10188    10210      +22     
==========================================
+ Hits         7015     7039      +24     
+ Misses       3173     3171       -2     

Impacted Files	Coverage Δ
bentoml/yatai/yatai_service_impl.py	52.65% <43.19%> (+0.90%)	⬆️
bentoml/yatai/db/__init__.py	80.88% <80.88%> (ø)
bentoml/yatai/db/stores/metadata.py	82.35% <82.35%> (ø)
bentoml/yatai/db/stores/label.py	84.52% <84.52%> (ø)
bentoml/yatai/db/stores/deployment.py	84.61% <84.61%> (ø)
bentoml/yatai/client/bento_repository_api.py	63.42% <100.00%> (ø)
bentoml/yatai/db/base.py	100.00% <100.00%> (ø)
...grations/versions/719dd2aacc9a_add_labels_table.py	60.00% <100.00%> (ø)
bentoml/yatai/validator/deployment_pb_validator.py	75.67% <100.00%> (ø)
... and 4 more

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