Implement (*Instance).Restarted
Retrieve restarts in GetInstance
Change behaviour of getFile to work with NoEnt

By return the file even on error, we open up the possibility for
retries or 'set' operations in case of ErrNoEnt.
Test (*Instance).Restarted
Test GetInstance with restarts
Merge pull request #87 from soundcloud/feature/instance-restarts

Keep track of instance restarts in coordinator