Remove network_event_loop instance in close()

Summary:
> 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.

This is a backport of Core [[bitcoin/bitcoin#17288 | PR17288]] [1/7]
bitcoin/bitcoin@ede8b76

Test Plan: `ninja && ninja check-functional`

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8241

Author: jachiang

test/functional/test_framework/mininode.py

@@ -571,6 +571,8 @@ def close(self, timeout=10):
                    timeout=timeout)
         self.network_event_loop.close()
         self.join(timeout)
+        # Safe to remove event loop.
+        NetworkThread.network_event_loop = None
 
 
 class P2PDataStore(P2PInterface):