CORE: UNITY: SPV: Implement an SPV specific AllocateShadowAccountsIfNeeded optimisation
CORE: UNITY: SPV: Implement an SPV specific AllocateShadowAccountsIfNeeded optimisation

If AllocateShadowAccountsIfNeeded allocates accounts too early in the sync it inteferes with the static filters (too many false positives); so we hold off on allowing AllocateShadowAccountsIfNeeded to operate until after we hit our first transaction, or after initial sync is done, whichever is first

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