refactor: Declare g_zmq_notification_interface as unique_ptr

Summary:
Ensures better memory safety for this global. This came up during
discussion of the following commit, but is not strictly required for its
implementation.

This is a partial backport of core#27125
https://github.com/bitcoin/bitcoin/pull/27125/commits/8ed4ff8e05d61a8e954d72cebdc2e1d1ab24fb84

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16182