Symptoms:
> app/control_router.go:44:38: error strings should not be capitalized or end with punctuation or a newline

This is blocking the build on:
https://circleci.com/gh/kinvolk/scope/363

- Add interfaces to allow for alternative implementations for Collector, ControlRouter
  and PipeRouter.
- Pass contexts on http handlers to these interfaces.  Although not used by the current
  (local, in-memory) implementations, the idea is this will be used to pass headers to
  implementations which support multitenancy (by, for instance, putting an authenticating
  reverse proxy in form of the app, and then inspecting the headers of the request for
  a used id).