ContextStack should be threadsafe

[memlucky71]

With each request in the web, the request is added to a request queue. Then, Thread pool chooses a thread to serve the request and send back the response.
Finally, this thread is added to the thread pool. In nanohttp, whole the concept is managed through the Context. In nanohttp, it is possible to make nested context. For each request, a thread stack is created.
Since two requests can use a single context, this can run into problems. So the context should be handled thread safe using thread_local.