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/example/main.go

@@ -94,7 +94,7 @@ func server() {
 
 	srv, serve := grpchelper.NewServer(authListener, clientIdentityKey, log, nil)
 
-	svc := &greeter{"hello "}
+	svc := &greeter{prepend: "hello "}
 	pdu.RegisterGreeterServer(srv, svc)
 
 	if err := serve(); err != nil {
@@ -103,6 +103,7 @@ func server() {
 }
 
 type greeter struct {
+	pdu.UnsafeGreeterServer
 	prepend string
 }