Update book_changes RPC to reduce latency, add "validated" field, and accept shortcut strings (current, closed, validated) for ledger_index.
`"validated": true` indicates that the transaction has been included in a validated ledger so the result of the transaction is immutable.
Fix#5033Fix#5034Fix#5035Fix#5036
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Co-authored-by: Bronek Kozicki <brok@incorrekt.com>