[backport#16774]Avoid unnecessary "Synchronizing blockheaders" log messages

Summary:
This prevents "Synchronizing blockeaders" from being displayed multiple times for the same block.
This is a backport of Core PR16774

Test Plan: ninja && ninja check`

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7634