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...

util/chunking.go

@@ -26,7 +26,7 @@ func NewUnchunker(conn io.Reader) *Unchunker {
 func (c *Unchunker) Read(b []byte) (n int, err error) {
 
 	if c.finishErr != nil {
-		return 0, err
+		return 0, c.finishErr
 	}
 
 	if c.remainingChunkBytes == 0 {
@@ -68,6 +68,19 @@ func (c *Unchunker) Read(b []byte) (n int, err error) {
 
 }
 
+func (c *Unchunker) Close() (err error) {
+
+	buf := make([]byte, 4096)
+	for err == nil {
+		_, err = c.Read(buf)
+		if err == io.EOF {
+			err = nil
+			break
+		}
+	}
+	return
+}
+
 func min(a, b int) int {
 	if a < b {
 		return a