Describe the bug
A clear and concise description of what the bug is.

NestedDiagnosticContext& NestedDiagnosticContext::current()
{
	static NestedDiagnosticContext ndc;
	return ndc;
}

It should use Thread Local Storage to store the NDC context. Not a singleton.

Please add relevant environment information:

• Windows 10
• poco-1.13.3