Feature/parallel migrations

[Nagelfar]

Using just IsolationLevel.Serializiable is not sufficient when running migrations in parallel.

• For Postgres the simplest solution seems to be using and advisory transaction lock.
• For SqlServer using Serializable is dangerous with regards to executed statements in the migration scripts
  Using with (SERIALIZABLE) and a retry mechanism worked in a stable way
• Not sure if parallel migrations is relevant with SqlLite

[coveralls]

Pull Request Test Coverage Report for Build 31

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-3.5%) to 96.471%

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Files with Coverage Reduction	New Missed Lines	%
C/projects/plainsql-migrations/PlainSql.Migrations/Migrator.cs	6	94.83%

Totals
Change from base Build 30:	-3.5%
Covered Lines:	164
Relevant Lines:	170

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💛 - Coveralls

[christophwalcher]

@Nagelfar @danielweller-swp still relevant?

[Nagelfar]

Relevant as in 'there is a problem with concurrent execution' - then I assume yes.
But not sure if it needs to be solved technical wise, or if updating the readme suffices.