Fix: port conflict with --debug flag

[bojiang]

Description

Fix: #848

Motivation and Context

When starting flask app with debug flag, it will launch a child process and restart to watch file changes, which spawns another batching server.

ref: https://flask.palletsprojects.com/en/1.1.x/server/#in-code

The reason for this is that due to how the reload mechanism works there are some bizarre side-effects (like executing certain code twice, sometimes crashing without message or dying when a syntax or import error happens).

How Has This Been Tested?

Run locally.

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

[codecov]

Codecov Report

Merging #858 into master will increase coverage by 2.46%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
+ Coverage   54.76%   57.22%   +2.46%     
==========================================
  Files         120      117       -3     
  Lines        9001     8085     -916     
==========================================
- Hits         4929     4627     -302     
+ Misses       4072     3458     -614     

Impacted Files	Coverage Δ
bentoml/marshal/marshal.py	29.57% <0.00%> (-0.21%)	⬇️
bentoml/saved_bundle/templates.py	100.00% <ø> (ø)
bentoml/server/bento_api_server.py	76.64% <0.00%> (ø)
bentoml/utils/cloudpickle.py
bentoml/_version.py
bentoml/adapters/pytorch_tensor_input.py
bentoml/configuration/__init__.py	73.80% <0.00%> (+2.38%)	⬆️
bentoml/saved_bundle/config.py	88.23% <0.00%> (+2.52%)	⬆️

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[parano]

Thanks for fixing this @bojiang!