[db-sync] Fix potential lost update

[geropl]

There is a gap in db-sync that might lead to lost updates/inserts in some scenarios. The details are outlined in this doc.

While we are in the process of #9198 we still intend to use it to ensure data quality before we migrate to a different system. So we should find a pragmatic way to fix this. As part of the deprecation we need to come up with tools to measure gaps like this in general.