ncp: Various changes in order to better support address change events (#165)

There are two logical sets of changes presented here:

* core/thread/mle: Follow remove/change/add pattern for unicast addresses

  The goal here is to avoid modifying an address after it has been added to
  the netif. That way we get reliable removal/insertion eventing. This
  pattern has other advantages which will likely become apparent later.

* openthread-types: Add `OT_IP6_ML_ADDR_CHANGED` flag

  This allows us to know precisely when the mesh local address has changed
  its value.

include/openthread-types.h

@@ -221,7 +221,9 @@ enum
     OT_NET_KEY_SEQUENCE     = 1 << 5,  ///< Thread Key Sequence changed
 
     OT_THREAD_CHILD_ADDED   = 1 << 6,  ///< Child was added
-    OT_THREAD_CHILD_REMOVED = 1 << 7   ///< Child was removed
+    OT_THREAD_CHILD_REMOVED = 1 << 7,  ///< Child was removed
+
+    OT_IP6_ML_ADDR_CHANGED  = 1 << 8,  ///< The mesh-local address has changed
 };
 
 /**