Follow up for #18183 and PR #18771.

Decide when to clear mutableSourceEagerHydrationData. We don't currently have a way to detect when all outstanding hydration work is finished. (It may never finish.) This means that in use cases where the version is the entire sore (e.g. an immutable store without a version "number") we would retain a reference to that store, preventing it from being GCed.