Use protoc-gen-go-grpc to generate protobuf files

[alexvanin]

With new protubuf API v2 there is a updated protoc-gen-go-grpc package to generate GRPC code. It should be updated one day with something like this:

protoc \
	--proto_path=.:./vendor:./vendor/github.com/nspcc-dev/neofs-api:/usr/local/include \
	--go_out=. --go_opt=paths=source_relative \
	--go-grpc_opt=require_unimplemented_servers=false \
	--go-grpc_out=. --go-grpc_opt=paths=source_relative $$f; \

See https://github.com/grpc/grpc-go/blob/master/cmd/protoc-gen-go-grpc/README.md for details on require_unimplemented_servers=false option.

Based on #173