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/rpc_client.go

@@ -172,14 +172,14 @@ func (c *Client) WaitForConnectivity(ctx context.Context) error {
 	}
 	go func() {
 		defer wg.Done()
-		ctrl, ctrlErr := c.controlClient.Ping(ctx, &req, grpc.FailFast(false))
+		ctrl, ctrlErr := c.controlClient.Ping(ctx, &req, grpc.WaitForReady(true))
 		checkRes(ctrl, ctrlErr, loggers.Control, &ctrlOk)
 	}()
 	go func() {
 		defer wg.Done()
 		for ctx.Err() == nil {
 			data, dataErr := c.dataClient.ReqPing(ctx, &req)
-			// dataClient uses transport.Connecter, which doesn't expose FailFast(false)
+			// dataClient uses transport.Connecter, which doesn't expose WaitForReady(true)
 			// => we need to mask dial timeouts
 			if err, ok := dataErr.(interface{ Temporary() bool }); ok && err.Temporary() {
 				// Rate-limit pings here in case Temporary() is a mis-classification