Remove known broken code & cleanup

[grigi]

Since pooled connections are known broken, This PR removes it, and cleans up the code, so that it will be easier to re-add it plainly.

[coveralls]

Pull Request Test Coverage Report for Build 337

• 13 of 40 (32.5%) changed or added relevant lines in 6 files are covered.
• 225 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+1.6%) to 93.133%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/backends/asyncpg/client.py	0	13	0.0%
tortoise/backends/mysql/client.py	0	14	0.0%

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/asyncpg/init.py	2	100.0%
tortoise/models.py	2	94.28%
tortoise/backends/mysql/init.py	2	100.0%
tortoise/backends/asyncpg/executor.py	6	100.0%
tortoise/backends/asyncpg/schema_generator.py	8	100.0%
tortoise/backends/mysql/schema_generator.py	13	100.0%
tortoise/backends/mysql/executor.py	21	100.0%
tortoise/backends/asyncpg/client.py	85	99.06%
tortoise/backends/mysql/client.py	86	99.11%

Totals
Change from base Build 328:	1.6%
Covered Lines:	2011
Relevant Lines:	2109

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💛 - Coveralls

[grigi]

Ok, so at this stage I fixed up the test runner to minimise re-connection, and do better resource isolation. And even fixed some resource leakage.
Now SQLite :memory: databases work for testing as we persist the connection correctly.
Hopefully this should significantly reduce the noise when trying to trace what is going on with connection pools.

[grigi]

rebased off master

[grigi]

This PR is getting a bit bit and confused. I removed a lot of code, then added a lot of code... so it just looks like churn
There is 4 things here:

1. Remove connection pools and related broken code
2. Change the config parser to be more generic and pass extra info in.
3. Add lots of type annotations
4. Add some tests

I feel I should break this in at least two PRs.

[grigi]

@Zeliboba5 Please review.