Make Bounds Checking a Runtime Toggle

[JPWatson]

Moved from real-logic/simple-binary-encoding#492 (@juddgaddie)

Like Java, C# (unlike c/c++) is typically not compiled from sources for that reason I think the bounds checking would be better served with a runtime toggle we could use system environment variables (the same approach as c or c++ code uses for runtime configuration).

I also would think the default build that goes to Nuget should have bounds checking enabled by default.

Example of what I mean here: https://gist.github.com/juddgaddie/eeafbbb195a332bf72808748cdcd4336

[JPWatson]

@juddgaddie Yep I'm on board with that in theory, I just have two reservations.

1. Would like to make sure there is no performance penalty (i.e. the JIT correctly optimises).
2. The environment variables for configuration is less idiomatic in .NET (but I can't think of any alternatives).

[juddgaddie]

As an alternative to 2. we could move Adaptive.Aeron/Config.cs into Adaptive.Agrona and then use the -D command line args. This would be more consistent with both the Java implementation and the .NET aeron port.