Remove network_event_loop instance in close()

The asyncio.new_event_loop() instance is now removed from the NetworkThread
class during shutdown. This enables a NetworkThread instance to be restarted
after being closed. The current NetworkThread class guards against an existing
new_event_loop during initialization.

test/functional/test_framework/mininode.py

@@ -478,7 +478,8 @@ class NetworkThread(threading.Thread):
         wait_until(lambda: not self.network_event_loop.is_running(), timeout=timeout)
         self.network_event_loop.close()
         self.join(timeout)
-
+        # Safe to remove event loop.
+        NetworkThread.network_event_loop = None
 
 class P2PDataStore(P2PInterface):
     """A P2P data store class.