AP_Mission: ensure cache coherence on mission resume

- when resuming a mission we should read the value from storage of the same index to ensure we're not continuing an old cached version of the mission item.
- inherent problem: DO commands will continue and will be unaffected unless the new mission has a different DO_ command structure. If so, a set_current_cmd() or reset() should be issued by the GCS.

libraries/AP_Mission/AP_Mission.cpp

@@ -91,6 +91,11 @@ void AP_Mission::resume()
         }
     }
 
+    // ensure cache coherence
+    // don't bother checking if the read is successful. If it fails _nav_cmd
+    // won't change and we'll continue flying the old cached command.
+    read_cmd_from_storage(_nav_cmd.index, _nav_cmd);
+
     // restart active navigation command. We run these on resume()
     // regardless of whether the mission was stopped, as we may be
     // re-entering AUTO mode and the nav_cmd callback needs to be run