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.

replication/logic/pdu/pdu_test.go

@@ -10,7 +10,7 @@ import (
 func TestFilesystemVersion_RelName(t *testing.T) {
 
 	type TestCase struct {
-		In    FilesystemVersion
+		In    *FilesystemVersion
 		Out   string
 		Panic bool
 	}
@@ -18,7 +18,7 @@ func TestFilesystemVersion_RelName(t *testing.T) {
 	creat := FilesystemVersionCreation(time.Now())
 	tcs := []TestCase{
 		{
-			In: FilesystemVersion{
+			In: &FilesystemVersion{
 				Type:     FilesystemVersion_Snapshot,
 				Name:     "foobar",
 				Creation: creat,
@@ -26,7 +26,7 @@ func TestFilesystemVersion_RelName(t *testing.T) {
 			Out: "@foobar",
 		},
 		{
-			In: FilesystemVersion{
+			In: &FilesystemVersion{
 				Type:     FilesystemVersion_Bookmark,
 				Name:     "foobar",
 				Creation: creat,
@@ -34,7 +34,7 @@ func TestFilesystemVersion_RelName(t *testing.T) {
 			Out: "#foobar",
 		},
 		{
-			In: FilesystemVersion{
+			In: &FilesystemVersion{
 				Type:     2342,
 				Name:     "foobar",
 				Creation: creat,