Implement transaction savepoints

[henadzit]

Description

This PR:

• Implements savepoints for Postgres, MySQL, MSSQL and Sqlite
• Improves test isolation. Since Tortoise.init makes global changes, in some cases test runs were impacting each other.
• Updates the docs and add a warning about starting concurrent tasks from within a transaction and starting nested transactions inside it.
• Updates CHANGELOG to plan for a major release 0.23 since the transaction behavior is going to change

Motivation and Context

Before the changes any exception within a transaction block, even nested, would rollback the whole transaction. Savepoints allow more granularity when handling errors:

    async with in_transaction():
        await MyModel.create(name='foo')
        try:
            # this block will create a savepoint
            async with in_transaction():
                await MyModel.create(name='bar')
                # this will rollback to the savepoint, meaning that
                # the 'bar' record will not be created, however,
                # 'foo' will be created
                raise Exception()
        except Exception:
            pass

How Has This Been Tested?

• make ci
• running local app

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 12409678196

Details

• 123 of 144 (85.42%) changed or added relevant lines in 9 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.3%) to 90.14%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/backends/oracle/client.py	1	2	50.0%
tortoise/backends/asyncpg/client.py	11	13	84.62%
tortoise/backends/psycopg/client.py	18	20	90.0%
tortoise/backends/odbc/client.py	4	7	57.14%
tortoise/backends/mssql/client.py	26	30	86.67%
tortoise/backends/sqlite/client.py	21	25	84.0%
tortoise/backends/mysql/client.py	26	31	83.87%

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/odbc/client.py	1	96.94%
tortoise/init.py	2	94.81%

Totals
Change from base Build 12342471369:	-0.3%
Covered Lines:	6368
Relevant Lines:	6949

---

💛 - Coveralls

[abondar]

Thanks, it's cool feature

[abondar]

Shouldn't it be 0.23.0?

[henadzit]

Fixed.

[abondar]

2 times for SQlite?

[henadzit]

Fixed :)