Improve SDS service at Citadel agent

[JimmyCYJ]

Bug description
Currently there are some improvements required at Citadel agent.

1. Make sure only one push is done per SDS request. We don't check push numbers for each SDS request, and we need to avoid multiple pushes.
2. Handle nonce in SDS request properly. This field is ignored by Citadel agent, only version info and token are checked.
3. Handle error_detail in a SDS request, we can log the error and add metrics for this error. Better to make this behavior consistent with Pilot.

Affected product area (please put an X in all that apply)

[ ] Configuration Infrastructure
[ ] Docs
[ ] Installation
[ ] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[X] Security
[ ] Test and Release
[ ] User Experience
[ ] Developer Infrastructure

#13439

[JimmyCYJ]

cc @quanjielin @myidpt

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 90 days. It will be closed in the next 30 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[istio-policy-bot]

🚧 This issue or pull request has been closed due to not having had activity from an Istio team member since 2019-07-25. If you feel this issue or pull request deserves attention, please reopen the issue. Please see this wiki page for more information. Thank you for your contributions.

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