Decimal toString() calls should include InvariantCulture

[robinabganpat]

Describe the bug
Currently on systems with an European culture is set not using the InvariantCulture might result in Decimal.ToString() to result in something like: "123,45" instead of "123.45". This then results in a parsing error when using in BigDecimal.Parse() for example.

To Reproduce
Steps to reproduce the behavior:

1. Set your system's culture to something like 'nl-NL' (so your system's decimal becomes a comma).
2. Run a project which uses the RPC Server (for example: neo-express).
3. See error logging for format exception.

Expected behavior
Independent of the culture of the system where NEO projects are ran, parsing should work as expected.

Screenshots
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Platform:

• OS: Windows 10 x64
• Version: v3.0.0-preview4