netty: Use await instead of sync
netty: Use await instead of sync

We only care about when closing is done, not whether it is successful or not.
If there's a failure, we're already going to log a warning. Use await to avoid
throwing unexpectedly.

dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,ejona86/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,grpc/grpc-java