Fix the logic for Context class with ThreadLocal

In the Context class, it has a ThreadLocal with type ContextStack which suppose to contain list of contexts created.

However, since Click is using Servlet mechanism, each request comes in is processed by a separate thread so no need to use ContextStack to store the Context instance.

By definition of Context, it is a request scope instance that is used manage context during a request to Click servlet. So by just using ThreadLocal<Context> it could could serve the purpose of having a single instance of Context during the life of the request to Click.