Update to protobuf v1.25 and grpc 1.35; bump CI to go1.12

From:
github.com/golang/protobuf v1.3.2
google.golang.org/grpc v1.17.0

To:
github.com/golang/protobuf v1.4.3
google.golang.org/grpc v1.35.0
google.golang.org/protobuf v1.25.0

About the two protobuf packages:
https://developers.google.com/protocol-buffers/docs/reference/go/faq
> Version v1.4.0 and higher of github.com/golang/protobuf wrap the new
implementation and permit programs to adopt the new API incrementally. For
example, the well-known types defined in github.com/golang/protobuf/ptypes are
simply aliases of those defined in the newer module. Thus,
google.golang.org/protobuf/types/known/emptypb and
github.com/golang/protobuf/ptypes/empty may be used interchangeably.

Notable Code Changes in zrepl:
- generate protobufs now contain a mutex so we can't copy them by value
  anymore
- grpc.WithDialer is deprecated => use grpc.WithContextDialer instead

Go1.12 is now actually required by some of the dependencies.

rpc/grpcclientidentity/grpchelper/authlistener_grpc_adaptor_wrapper.go

@@ -34,8 +34,10 @@ func ClientConn(cn transport.Connecter, log Logger) *grpc.ClientConn {
 		Timeout:             KeepalivePeerTimeout,
 		PermitWithoutStream: true,
 	})
-	dialerOption := grpc.WithDialer(grpcclientidentity.NewDialer(log, cn))
+	dialerOption := grpc.WithContextDialer(grpcclientidentity.NewDialer(log, cn))
 	cred := grpc.WithTransportCredentials(grpcclientidentity.NewTransportCredentials(log))
+	// we use context.Background without a timeout here because we don't set grpc.WithBlock
+	// => docs:  "In the non-blocking case, the ctx does not act against the connection. It only controls the setup steps."
 	cc, err := grpc.DialContext(context.Background(), "doesn't matter done by dialer", dialerOption, cred, ka)
 	if err != nil {
 		log.WithError(err).Error("cannot create gRPC client conn (non-blocking)")