gui: Throttle GUI update pace when -reindex
Summary:
With this PR, the GUI does not freeze when a user runs: $ ./src/qt/bitcoin-qt -reindex
Backport of core PR18121.
Test Plan:
ninja all check ./src/qt/bitcoin-qt -reindex
Check the GUI sync progress no longer causes epileptic seizure to the
user.
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8323