Drop runner API from visor.Runner

Visor lets you register, unregister, and discover runners. The runner's
API should live outside of visor.
Restarted() accepts InsRestarts

Restarted() no longer reads the current value to apply a delta for each
kind of restart.
Remove branch for missing OOM field in restart file
Drop visor/net package
Merge pull request #110 from soundcloud/update-runner-interface

Drop runner API from visor.Runner