Encode client-broker messages as json in HTTP body

Send the client poll request and response in a json-encoded format in
the HTTP request body rather than sending the data in HTTP headers. This
will pave the way for using domain-fronting alternatives for the
Snowflake rendezvous.

broker/snowflake-heap.go

@@ -13,7 +13,7 @@ type Snowflake struct {
 	proxyType     string
 	natType       string
 	offerChannel  chan *ClientOffer
-	answerChannel chan []byte
+	answerChannel chan string
 	clients       int
 	index         int
 }