Add registry test for timeOfLastUpdate
Add time_of_last_registry_update to varz
Add test for varz time_of_last_registry_update in router

[#53722497]
Router varz reports seconds since last registry update

[#53722497]
Merge pull request #7 from cloudfoundry/varz-registry-update-time

Varz registry update time (yay, tests passed on Travis) + yetis