config: /config_dump shows rejected RDS config vs the last valid RDS config envoy is running

Description:
When triaging an invalid RDS config on our fleet, /config_dump showed the version sha of the bad config, rather than version sha envoy was running (which was the last valid configuration). So when a new box came up that was not able to serve traffic, config_dump showed the same sha, making it very difficult to debug.

We ended up realizing RDS was being rejected via startup logs and update_rejected stat. But we should have solved it faster of config_dump showed the last valid version sha.

Repro steps:
Have the control plane send RDS config with a duplicate domain.

Logs:

[2019-08-13 22:21:07.185][31248][warning][config] [external/envoy/source/common/config/grpc_mux_subscription_impl.cc:72] gRPC config for type.googleapis.com/envoy.api.v2.RouteConfiguration rejected: Only unique values for domains are permitted. Duplicate entry of domain maptiles-staging.lyft.net