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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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