Ensure consistent transaction name for unknown-route HTTP requests

[axw]

We should ensure consistency with other agents, and set the transaction name to "<METHOD> unknown route" for incoming HTTP requests, when the route cannot be determined.

[axw]

This is a little bit complicated for gorilla/mux, since its middleware isn't invoked at all when a route cannot be matched. To support this we'll need to override the Router.NotFoundHandler and Router.MethodNotAllowedHandler fields (see https://godoc.org/github.com/gorilla/mux#Router).

[axw]

Likewise for httprouter.