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.
Turns out that files with just service methods don't get loaded into the
descriptor pool automatically in C++. So, needed to have some messages
in ads.proto. Turns out this was a good opportunity to move some of the
messages that were related to discovery out of base.proto.
This shouldn't break the API, since everything is in the envoy.api.v2
packge space.
htuch