AP_HAL: defer calls to safety_state switch to queue an async attempt.

- this allows for auto-retries if the state does not set correctly

libraries/AP_HAL/RCOutput.h

@@ -101,6 +101,11 @@ public:
      */
     virtual void     force_safety_off(void) {}
 
+    /*
+      If we support async sends (px4), this will force it to be serviced immediately
+     */
+    virtual void     force_safety_no_wait(void) {}
+
     /*
       setup scaling of ESC output for ESCs that can output a
       percentage of power (such as UAVCAN ESCs). The values are in