Adding a /healthz url to vttablet.

It reflects the last result of the health check.
Adding integration tests to confirm it's wired up properly.
Adding a health check for query service in the right state.

CHanging the health check interface to pass a flag indicating
if QS should be running or not.
Now checking tabletserver.IsHealthy in health check.
Fixing health links in vttablet status page.
Merge branch 'master' into replication
Merge pull request #232 from youtube/replication

Replication