ConfigServer: File system watcher does not update internal config

[mustafaquraish]

It seems that the old_value field in the file system watcher is never updated to be the new one even if we have a change. This means that:

• If we change a value from X to Y, we can never change it back to X, since ConfigServer thinks the value is always X
• If we change a value from X to Y, and save the file repeatedly without any changes, messages are constantly sent to all listeners even though nothing in the config has changed.

I'm happy to attempt to fix this, but I'm not 100% sure how to go about this, any advice would be great!

Here's a demonstration video (Using #9639, with the fix from 317f55b applied): Video

[mustafaquraish]

Edit: Upon some further testing, it seems 317f55bb has not fixed the issue.