Optimize field conversion to database format

[henadzit]

Description

• Remove TO_DB_OVERRIDE that is no longer needed after parametrization changes. The only exception is SQLite for which sqlite3.register_adapter was added - this is the same approach used by Django.
• Get rid of BaseExecutor.column_map. It was only required because of TO_DB_OVERRIDE.
• Cache timezone.get_use_tz, timezone.get_timezone and timezone.get_default_timezone. Since these are called for each field with auto_now or auto_now_add, it provides a significant speedup.

Motivation and Context

This PR improves the performance of field value conversion to the DB format (see the detailed report in the comments):

• 52% improvement in the test_bulk_create_many_fields benchmark
• 13% improvement in test_bulk_create_few_fields
• 9% improvement in test_create_many_fields

The speedup is primarily achieved by the removal of self.model._meta.db from the Field. to_db_value code. The db property gets the current connection from ContextVar and it was a relatively slow operation to do it for each field for each object in bulk_create.

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.

[codspeed-hq]

CodSpeed Performance Report

Merging #1840 will improve performances by 52.82%

_{Comparing henadzit:chore/to_db_value_unused-code (6562ce4) with develop (8b5ac14)}

Summary

⚡ 2 improvements
✅ 6 untouched benchmarks

Benchmarks breakdown

	Benchmark	develop	henadzit:chore/to_db_value_unused-code	Change
⚡	test_bulk_create_few_fields	3 ms	2.6 ms	+13.12%
⚡	test_bulk_create_many_fields	17.3 ms	11.3 ms	+52.82%

[coveralls]

Pull Request Test Coverage Report for Build 12631809925

Details

• 20 of 20 (100.0%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 90.252%

---

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/mssql/executor.py	1	83.33%

Totals
Change from base Build 12617761088:	0.1%
Covered Lines:	6427
Relevant Lines:	7005

---

💛 - Coveralls

[henadzit]

@abondar any chance you can have a look at this? Thanks!

[abondar]

Good job!