zmq: remove LOCK(cs_main) from NotifyBlock

Summary:

The second commit is from a suggestion on the original PR by jonatack to remove the unnecessary LOCK(cs_main) in the zmq notifier's NotifyBlock.

This is a partial backport of core#26308
https://github.com/bitcoin/bitcoin/pull/26308/commits/7d253c943f44612431be894b198ffb49ff988fff

Depends on D13052

Test Plan:
With Clang and DEBUG:
ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13053