Bump version to 0.6.4 for testing, integrate software updater auto-check into PacketDecoder decode path and main loop.

node/Constants.hpp

@@ -340,6 +340,11 @@ error_no_byte_order_defined;
  */
 #define ZT_UPDATE_MIN_INTERVAL 120000
 
+/**
+ * Maximum interval between attempts to do a software update
+ */
+#define ZT_UPDATE_MAX_INTERVAL 28800000
+
 /**
  * Update HTTP timeout in seconds
  */