Final performance restructure on Model constructor. A further 9~11% s…

[grigi]

A further 9~11% speedup.

I also tried to restructure the constructor to be more logically grouped and added comments.

[coveralls]

Pull Request Test Coverage Report for Build 204

• 20 of 21 (95.24%) changed or added relevant lines in 1 file are covered.
• 318 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.8%) to 91.547%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/models.py	20	21	95.24%

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/base/executor.py	1	95.56%
tortoise/backends/mysql/init.py	2	100.0%
tortoise/backends/asyncpg/init.py	2	100.0%
tortoise/models.py	3	94.04%
tortoise/backends/asyncpg/schema_generator.py	8	100.0%
tortoise/backends/base/client.py	8	91.07%
tortoise/backends/mysql/schema_generator.py	13	100.0%
tortoise/backends/asyncpg/executor.py	13	100.0%
tortoise/backends/mysql/executor.py	29	100.0%
tortoise/backends/asyncpg/client.py	117	80.37%

Totals
Change from base Build 202:	0.8%
Covered Lines:	730
Relevant Lines:	781

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💛 - Coveralls