* Simplify server lib responses so HTTP server components don't
have to unwrap protobuf Response types intended for gRPC uses
* Wrap server lib responses in protobuf Response types in the
rpc package
* Add gRPC SelectGroup and SelectProfile service endpoints
* HTTP and gRPC servers should both allow labels to be resolved to the
matching Group or Profile. Move logic to bootcfg core server.
* Add gRPC wrapper selectServer to avoid cluttering core server with
response types which are currently only useful for gRPC
* Remove api.Store and use storagepb.Store instead
* Remove api.Spec and use storagepb.Profile instead
* Switch from api.Group to storagepb.Group
* Move api.Group to config for YAML config decoding only
