discovery: empty list of last discovered nodes using ND

When a non-deep discovery is launched, all discovered nodes are added to a
list. But this list is never emptied that is clearly a memory leak.

This commit clears this list before starting a new discovery process and once
the discovery of a list of devices finishes.

#172

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>

digi/xbee/devices.py

@@ -7765,7 +7765,9 @@ def discover_devices(self, device_id_list):
         self.start_discovery_process()
         while self.is_discovery_running():
             time.sleep(0.1)
-        return list(filter(lambda x: x.get_node_id() in device_id_list, self.__last_search_dev_list))
+        discovered_devices = list(filter(lambda x: x.get_node_id() in device_id_list, self.__last_search_dev_list))
+        self.__last_search_dev_list.clear()
+        return discovered_devices
 
     def is_discovery_running(self):
         """
@@ -8815,6 +8817,7 @@ def __discover_devices_and_notify_callbacks(self, discover_network=False):
                 with a single 'ND'.
         """
         self._stop_event.clear()
+        self.__last_search_dev_list.clear()
         self.__discovering = True
         self.__discover_result = None