Mirror takes long time to start after upgrade

Issue Summary

When a mirror (single or farm) is upgraded to a newer version, it takes very long time to go from Authorized to Synchronizing state. The time taken is much longer than taken by a normal mirror restart. After the first mirror is started on a new version, then other mirrors in the farm do not take that long to go to Synchronizing state.

Steps to Reproduce

1. Given a primary instance with 10K or more repositories.
2. Setup a single mirror and use "Add all projects" option to sync all the repositories on mirror.
3. Wait for the mirror to sync all the repositories and running as normal.
4. Shutdown the mirror.
5. Install a new version (bugfix, minor or major version) or mirror.
6. Start the mirror and wait for it to go to Synchronizing state.

Expected Results

The mirror should go to Synchronizing state within 1-2 minutes after it goes to Authorized state.

Actual Results

The mirror is stuck in Authorized state for long time. The amount of time depends on the number of repositories synced on mirror.

Workaround

Restart the mirror after the following line is printed in the logs - 

2023-10-15 20:50:36,389 INFO  [Caesium-1-3] @57X4NRx1250x1366568052x2 c.a.b.i.m.m.UpstreamRegistrationJob Upgraded mirror add-on in https://stash.atlassian.com