Remove network_event_loop instance in close()
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 [[https://github.com/bitcoin/bitcoin/pull/17288 | PR17288]] [1/7]
https://github.com/bitcoin/bitcoin/pull/17288/commits/ede8b7608e115364b5bb12e7f39d662145733de6

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

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