Title: Envoy starts listening ports before it receive first RDS response

Description:
In our Istio environment, after Envoy received LDS response, it starts listening ports before it receives first RDS response, that may cause "404 NR" issue, especially when Istio pilot need a long time to calculate routes and put them to Envoy.

Looks like it's more reasonable to make Envoy listening ports after it receives the first RDS response. What do you think?