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/replication_logic.go

@@ -117,7 +117,7 @@ func (p *Planner) WaitForConnectivity(ctx context.Context) error {
 type Filesystem struct {
 	sender   Sender
 	receiver Receiver
-	policy   PlannerPolicy
+	policy   PlannerPolicy // immutable, it's .ReplicationConfig member is a pointer and copied into messages
 
 	Path                 string             // compat
 	receiverFS, senderFS *pdu.Filesystem    // receiverFS may be nil, senderFS never nil
@@ -569,7 +569,7 @@ func (s *Step) buildSendRequest(dryRun bool) (sr *pdu.SendReq) {
 		Encrypted:         s.encrypt.ToPDU(),
 		ResumeToken:       s.resumeToken,
 		DryRun:            dryRun,
-		ReplicationConfig: &s.parent.policy.ReplicationConfig,
+		ReplicationConfig: s.parent.policy.ReplicationConfig,
 	}
 	return sr
 }
@@ -614,7 +614,7 @@ func (s *Step) doReplication(ctx context.Context) error {
 		Filesystem:        fs,
 		To:                sr.GetTo(),
 		ClearResumeToken:  !sres.UsedResumeToken,
-		ReplicationConfig: &s.parent.policy.ReplicationConfig,
+		ReplicationConfig: s.parent.policy.ReplicationConfig,
 	}
 	log.Debug("initiate receive request")
 	_, err = s.receiver.Receive(ctx, rr, byteCountingStream)