rpc: chunk JSON parts of communication + refactoring

JSONDecoder was buffering more of connection data than just the JSON.
=> Unchunker didn't bother and just started unchunking.

While chaining JSONDecoder.Buffered() and the connection using
ChainedReader works, it's still not a clean architecture.

=> Every JSON message is now wrapped in a chunked stream
   (chunked and unchunked)
   => no special-cases
=> Keep ChainedReader, might be useful later on...

rpc/rpc_test.go

@@ -0,0 +1,33 @@
+package rpc
+
+import (
+	"bytes"
+	"github.com/stretchr/testify/assert"
+	"github.com/zrepl/zrepl/util"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestByteStreamRPCDecodeJSONError(t *testing.T) {
+
+	r := strings.NewReader("{'a':'aber'}")
+
+	var chunked bytes.Buffer
+	ch := util.NewChunker(r)
+	io.Copy(&chunked, &ch)
+
+	type SampleType struct {
+		A uint
+	}
+	var s SampleType
+	err := readChunkedJSON(&chunked, &s)
+	assert.NotNil(t, err)
+
+	_, ok := err.(ByteStreamRPCDecodeJSONError)
+	if !ok {
+		t.Errorf("expected ByteStreamRPCDecodeJSONError, got %t\n", err)
+		t.Errorf("%s\n", err)
+	}
+
+}