Upgrade protoc, protobuf, gRPC, Go and gRPC protoc plugins

%14.0 is a great chance to bump these important dependencies. I think it's 100% backwards compatible (the wire format is the same), but who knows what might happen.

One potentially breaking change is unix socket handling. Gitaly uses unix://some/path but it's not a valid naming scheme according to https://github.com/grpc/grpc/blob/master/doc/naming.md. Valid ones are: unix:/absolute/path, unix:relative/path or unix:///absolute/path. I have fixed most/all(?) of the issues related to that and Gitaly client still does the parsing itself (not delegating to gRPC resolver), so it should not be breaking, but something somewhere may not be accounted for. Needs proper testing and, as I said above, %14.0 is a good time to release something potentially breaking.