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.

daemon/job/active.go

@@ -160,7 +160,7 @@ func modePushFromConfig(g *config.Global, in *config.PushJob, jobID endpoint.Job
 
 	m.plannerPolicy = &logic.PlannerPolicy{
 		EncryptedSend:     logic.TriFromBool(in.Send.Encrypted),
-		ReplicationConfig: *replicationConfig,
+		ReplicationConfig: replicationConfig,
 	}
 
 	if m.snapper, err = snapper.FromConfig(g, m.senderConfig.FSF, in.Snapshotting); err != nil {
@@ -255,7 +255,7 @@ func modePullFromConfig(g *config.Global, in *config.PullJob, jobID endpoint.Job
 
 	m.plannerPolicy = &logic.PlannerPolicy{
 		EncryptedSend:     logic.DontCare,
-		ReplicationConfig: *replicationConfig,
+		ReplicationConfig: replicationConfig,
 	}
 
 	m.receiverConfig, err = buildReceiverConfig(in, jobID)