Make vtkLogger::ThreadName a thread_local variable

vtkLogger::Init() resets the ThreadName of the calling thread using the last used value of the static variable vtkLogger::ThreadName.

This means that if multiple threads call vtkLogger::SetThreadName(ThreadName) and then any of them calls vtkLogger::Init() it will mess up its name.

The attached example fails in current master.

This issue was found by naming multiple threads and then later calling vtkPythonInterpreter::Initialize() which internally calls vtkLogger::Init().

Making the global vtkLogger::ThreadName static thread_local instead of just static resolves the issue.

Visual Studio compiler emits

error C2492: 'vtkLogger::ThreadName': data with thread storage duration may not have dll interface

when combining static with thread_local , so the variable has been moved from the header file to the internal detail namespace.