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.

.circleci/config.yml

@@ -172,7 +172,7 @@ workflows:
           matrix: &quickcheck-go-matrix
             alias: quickcheck-go-matrix
             parameters:
-              goversion: [*latest-go-release, "1.11"]
+              goversion: [*latest-go-release, "1.12"]
               goos: ["linux", "freebsd"]
               goarch: ["amd64", "arm64"]
             exclude:
@@ -180,8 +180,8 @@ workflows:
               - goversion: *latest-go-release
                 goos: linux
                 goarch: amd64
-              # not supported by Go 1.11
-              - goversion: "1.11"
+              # not supported by Go 1.12
+              - goversion: "1.12"
                 goos: freebsd
                 goarch: arm64