init: Delay RPC block notif until warmup finished

Summary:
Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/2414ebc18b8bebf79c47e58a4293d0fc6420a811

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12576