Automatic merge from submit-queue (batch tested with PRs 44727, 45409, 44968, 45122, 45493)
Separate healthz server from metrics server in kube-proxy
From #14661, proposal is on kubernetes/community#552.
Couple bullet points as in commit:
- /healthz will be served on 0.0.0.0:10256 by default.
- /metrics and /proxyMode will be served on port 10249 as before.
- Healthz handler will verify timestamp in iptables mode.
/assign @nicksardo @bowei @thockin
**Release note**:
```release-note
NONE
```
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