Removal of residual NetworkThread state.

Author: jachiang

test/functional/test_framework/mininode.py

@@ -480,7 +480,8 @@ def close(self, timeout=10):
         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.