Fix registering of background updates for split main/state db

[clokep]

The background updates are being registered on an object that is for the state database, but the actual tables are on the main database. This just moves them to a different store that can access the right stuff.

I noticed this when trying to do a full schema dump cause I was curious what has changed since the last one.

Fixes #16054

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[erikjohnston]

Oops!