fix: derived store restarting when unsubscribed from another store with a shared ancestor

[jrouleau]

Fixes #8364

When a derived store was unsubscribed from another store sharing a common ancestor, the ancestors update loop would restart the store after it had already been stopped. This PR renames the existing inited boolean to started to be more clear about its function and resets it to false once the store has been stopped so sync() isn't run on a cold store.

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[dummdidumm]

Thank you!