Update gorouter

$ git shortlog 16c3ecd2..1678894e
David Sabeti and Pieter Noordhuis (5):
      Refactor registry operations
      Rename registerMessage -> registryMessage
      Combine lookup logic in registry
      Remove obsoleted code
      Update github.com/rcrowley/go-metrics

Change-Id: I83fa56a48d8c8e9b2fc22b37eebc553154187e25