grpc: add explicit nonce to DiscoveryResponse. (#172)
grpc: add explicit nonce to DiscoveryResponse. (#172)

This is a fix for a potential race that exists in the gRPC
implementation today. Namely:

1. Envoy sends a DiscoveryRequest at X with resources {A, B}
2. Management server responds with DiscoveryResponse at X+1.
3. Envoy decides (for whatever reason) to change resources before
   beceiving the X+1 response. It sends a DiscoveryRequest at X
   with {A, B, C}.
4. Management server treats the X DiscoveryRequest update as a NACK
   for the X+1 DiscoveryResponse.
jrajahalme/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,lyft/envoy-api,jrajahalme/envoy-api,rshriram/envoy-api,rshriram/envoy-api,rshriram/envoy-api,lyft/envoy-api,lyft/envoy-api