Stop prefetching invalid entries

[sisuresh]

The bulk load path ignores the invariants enforced on regular loads. For example, a trustline for the issuer cannot exist, so it cannot be loaded, but core still allows you to try prefetching that entry. We should stop prefetching invalid entries and enforce the invariants in bulk load as well. This would allow us to add https://github.com/stellar/stellar-core/pull/4189/ back in.

[anupsdf]

Related discussion in slack thread