gui: Throttle GUI update pace when -reindex
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 [[https://github.com/bitcoin/bitcoin/pull/18121 | 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

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc