Review current generated code

[odeheurles]

Current generated code is very close to Java API, which is not very idiomatic for a C# developer. We need to go through the generated code and decide how the different SBE constructs (fields, composites, bitsets, etc..) should be implemented and exposed in .NET.

Output of this task should be a list of the changes to apply in the code generation.

Some initial ideas:

• optional fields as Nullable<>
• set to be implemented a C# enum with [Flag]
• groups API is horrible, need to be looked at

[mattbarrett]

WrapForEncode and WrapForDecode can be improved. Doesn't read very well.

I think we can remove the non-idiomatic iteration completely, and just have a .net version that throws when its misused (different threads, same thread iterating more than once) for no allocation. If allocation is allowed, then perhaps a different implementation that is less constrained?