Fix a potential bug into the new implementation of the Jupyterhub Leader Election Sidecar Container. 

We should add a new Liveness Probe to check the potential fencing of the new go-client: https://github.com/kubernetes/client-go/blob/master/tools/leaderelection/leaderelection.go#L19

In other to accomplish that we should:

1. Update the sidecar image to add a new endpoint to check if the image think is the leader elected.
2. Add the liveness probe to check the actual leader elected and, if the image thinks it's the leader but it's not the global leader restart that container.