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## Problem
We saw the following scenario in staging:
1. Pod A starts up. Becomes leader and steps down the previous pod
cleanly.
2. Pod B starts up (deployment).
3. Step down request from pod B to pod A times out. Pod A did not manage
to stop its reconciliations within 10 seconds and exited with return
code 1
([code](7ba8519b43/storage_controller/src/service.rs (L8686-L8702))).
4. Pod B marks itself as the leader and finishes start-up
5. k8s restarts pod A
6. k8s marks pod B as ready
7. pod A sends step down request to pod A - this succeeds => pod A is
now the leader
8. k8s kills pod A because it thinks pod B is healthy and pod A is part
of the old replica set
We end up in a situation where the only pod we have (B) is stepped down
and attempts to forward requests to a leader that doesn't exist. k8s
can't detect that pod B is in a bad state since the /status endpoint
simply returns 200 hundred if the pod is running.
## Summary of changes
This PR includes a number of robustness improvements to the leadership
protocol:
* use a single step down task per controller
* add a new endpoint to be used as k8s liveness probe and check
leadership status there
* handle restarts explicitly (i.e. don't step yourself down)
* increase the step down retry count
* don't kill the process on long step down since k8s will just restart
it