precompute sizes and and simplify BasicProperties presence

[bollhals]

Proposed Changes

There are two changes in this PR.
1)
The size calculations for GetRequiredPayloadBufferSize & GetRequiredBufferSize are now as much precomputed as possible.
E.g.
Before

    public override int GetRequiredBufferSize()
    {
      int bufferSize = 0;
      bufferSize++; // _versionMajor in bytes
      bufferSize++; // _versionMinor in bytes
      bufferSize += WireFormatting.GetTableByteCount(_serverProperties); // _serverProperties in bytes
      bufferSize += 4 + _mechanisms.Length; // _mechanisms in bytes
      bufferSize += 4 + _locales.Length; // _locales in bytes
      return bufferSize;
    }

After

    public override int GetRequiredBufferSize()
    {
      int bufferSize = 10; // bytes for _versionMajor, _versionMinor, length of _mechanisms, length of _locales
      bufferSize += WireFormatting.GetTableByteCount(_serverProperties); // _serverProperties in bytes
      bufferSize += _mechanisms.Length; // _mechanisms in bytes
      bufferSize += _locales.Length; // _locales in bytes
      return bufferSize;
    }

Changed the BasicProperties internals of the present check.
Now whether or not a property is present, is not a field anymore, but a check against != default.

Difference: If a default value is assinged, it is considered not present. Also less fields, smaller memory footprint.

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
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• Cosmetic change (whitespace, formatting, etc)

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[michaelklishin]

@bollhals these are reasonable changes, thank you. We trust your judgement but for the sake of transparency, how do the allocation numbers change with this modification?

[bollhals]

Before

After

It's a 8 byte reduction per instance, not much but it's something. I was expecting more as there were 14 bools eliminated.

[michaelklishin]

Given the muted effectiveness of this change (looks like due to C# compiler optimizations?), let's keep this specific to 7.0. Of course, by 7.0 we might remove API generator entirely but only time will tell. We appreciate @bollhals' ongoing contributions either way 🥇