feat(mongo): refactor _fields

[Hieuzest]

• remove unused field field, use id to replace table
• trigger initialization on first time migration to this version
• store current fields to avoid migration on every startup

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.00%. Comparing base (b2d7cd6) to head (ca90c57).

Files with missing lines	Patch %	Lines
packages/mongo/src/index.ts	98.07%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   99.01%   99.00%   -0.02%     
==========================================
  Files          39       39              
  Lines       13638    13655      +17     
  Branches     3367     3370       +3     
==========================================
+ Hits        13504    13519      +15     
- Misses        134      136       +2     

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