Allow queries by start_seq=ledger_index + tx_index on /v2/accounts/account/payments

[offlinehacker]

When we are processing payments, we don't want to miss transactions, so we are paginating by start time, because this is currently the simplest way to do. We also store last ledger_index and tx_index to skip any transactions that could occur at the same time. It would be nice if it would be possible to query all account payments that start at some ledger_index + tx_index.