streamlineGlobalContext: Support resetting global context before handling request

In streamline, the global context will persist unless you reset it at the beginning of the request. This PR wraps the app.handle method to reset the context before continuing processing the request.

Edit by @aseemk: fixes #12.
I don't think we want to have the middleware send the response, as that's not a realistic use case.

E.g. for setting trace IDs, or request locales, or noting consistency requirements, etc., none of those middlewares would end the response. They would just write to the global context.