[C#] Convert benchmarks to BenchmarkDotNet

[szehetner]

As mentioned in #840, this converts the C# benchmark project to use BenchmarkDotNet instead of the handcrafted benchmark logic.

Notable changes:

• The target framework (for the benchmark project only) was changed from .NET Framework 4.5 to 4.6.1 as this is the minimum required by BenchmarkDotNet (needs to be netstandard 2.0 compatible).
• The output assembly was renamed to sbe-benchmarks (BenchmarkDotNet requires the assembly to have the same name as the .csproj file)

Benchmarks:

• MarketDataBenchmark is basically the same as the existing one, only the BenchmarkState class was merged into it.
• CarBenchmark is based on the car example, covering more features.

[mjpt777]

Note there are a lot of warnings after merging this PR in the benchmarks. The assigned values probably need to be consume to make the benchmark valid.