VERSION 0.6.11: Windows wrap-up work, NAT-t fixes

This version fixes a minor NAT traversal issue. In the past, NAT-t links had a timeout
but otherwise were preserved. This version makes them more ephemeral and invalidates
them on sleep/wake or changes in network configuration or environment.

This is because many NAT setups are very fragile with regard to hole punches, so the
past stickiness of links caused dead links to persist too long and break connectivity
between peers.

This is about 75% of what needs to be done to greatly improve robustness. The other 25%
involves detecting failed links or failed relays.

This version is also almost done for the Windows platform, moving us even closer to
binary release.

version.h

@@ -41,6 +41,6 @@
 /**
  * Revision
  */
-#define ZEROTIER_ONE_VERSION_REVISION 10
+#define ZEROTIER_ONE_VERSION_REVISION 11
 
 #endif