Switch default proxy STUN server to stun.l.google.com

This is the same default that the web-based proxies use. Proxies do not
need RFC 5780 compatible STUN servers.

doc/snowflake-proxy.1

@@ -29,7 +29,7 @@ websocket relay URL (default "wss://snowflake.torproject.net/")
 .HP
 \fB\-stun\fR string
 .IP
-stun URL (default "stun:stun.stunprotocol.org:3478")
+stun URL (default "stun:stun.l.google.com:19302")
 .HP
 \fB\-unsafe\-logging\fR
 .IP

proxy/README.md

@@ -48,7 +48,7 @@ Usage of ./proxy:
   -relay string
         websocket relay URL (default "wss://snowflake.bamsoftware.com/")
   -stun string
-        STUN URL (default "stun:stun.stunprotocol.org:3478")
+        stun URL (default "stun:stun.l.google.com:19302")
   -summary-interval duration
         the time interval to output summary, 0s disables summaries. Valid time units are "s", "m", "h".  (default 1h0m0s)
   -unsafe-logging

proxy/lib/snowflake.go

@@ -7,7 +7,7 @@ will be set to the defined defaults.
 
 	proxy := snowflake_proxy.SnowflakeProxy{
 		BrokerURL: "https://snowflake-broker.example.com",
-		STUNURL: "stun:stun.stunprotocol.org:3478",
+		STUNURL: "stun:stun.l.google.com:19302",
 		// ...
 	}
 
@@ -57,7 +57,7 @@ const DefaultNATProbeURL = "https://snowflake-broker.torproject.net:8443/probe"
 
 const DefaultRelayURL = "wss://snowflake.bamsoftware.com/"
 
-const DefaultSTUNURL = "stun:stun.stunprotocol.org:3478"
+const DefaultSTUNURL = "stun:stun.l.google.com:19302"
 const DefaultProxyType = "standalone"
 const pollInterval = 5 * time.Second