refactor: use db queryset to reduce duplicated code

[waketzheng]

Description

There are server repeat code segments in tortoise/models.py as follows:

db = using_db or cls._choose_db()
cls._meta.manager.get_queryset().using_db(db)

Motivation and Context

Add a new classmethod _db_queryset to Model

How Has This Been Tested?

make ci

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 9513948759

Details

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.003%) to 88.944%

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Totals
Change from base Build 9479020180:	-0.003%
Covered Lines:	5853
Relevant Lines:	6481

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9537193587

Details

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 89.049%

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Totals
Change from base Build 9534241702:	-0.004%
Covered Lines:	5868
Relevant Lines:	6492

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9537259684

Details

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 89.049%

---

Totals
Change from base Build 9534241702:	-0.004%
Covered Lines:	5868
Relevant Lines:	6492

---

💛 - Coveralls