Performance on insert many

[mkolumb]

Hello, I noticed some performance issue with insert many on big collection.
It looks like there are few reasons

• unnecessary cast to list
• using string operator instead of string builder
• compilation as .net standard (or maybe another problem with build, insert from collection with 50k rows and 3 columns takes 15 seconds when using nuget 2.3.7 package, around 1.2 seconds when compiled directly for .net core 3.1)

I fixed first two things, but I didn't change target frameworks, I think it would be good to compile sql kata as multiple frameworks, including .net core 3.1, .net 5.0 and .net 6.0

What I changed

• Add insert many overload
• Use dictionary in insert clause
• Fixed sql server compiler test runner (now every test class have their own instance and legacy pagination is used in tests)
• Improved string build performance

[mkolumb]

@ahmad-moussawi Any chance to review?

[mkolumb]

@ahmad-moussawi