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.
Nice.

Minor: let's clarify the "return global context from middleware" if you agree with my feedback that the middleware shouldn't be returning. E.g. "set global context from middleware and return from route handler".