RIPD-1847 fix select query condition

High Level Overview of Change

perf: improve account_tx SQL query:

The witness server makes heavily use of the account_tx RPC command. Perf testing showed that the SQL query used by account_tx became unacceptably slow when the DB was large and there was a marker parameter. The plan for the query showed only indexed reads. This appears to be an issue with the internal SQLite optimizer. This patch rewrote the query to use UNION instead of OR and significantly improves performance. See RXI-896 and RIPD-1847 for more details.

Example

was:

SELECT AccountTransactions.LedgerSeq,AccountTransactions.TxnSeq,
  Status,RawTxn,TxnMeta
  FROM AccountTransactions, Transactions WHERE
  (AccountTransactions.TransID = Transactions.TransID AND
  AccountTransactions.Account = 'rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh' AND
  AccountTransactions.LedgerSeq BETWEEN '415398' AND '415441')
  OR
  (AccountTransactions.TransID = Transactions.TransID AND
  AccountTransactions.Account = 'rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh' AND
  AccountTransactions.LedgerSeq = '415442' AND
  AccountTransactions.TxnSeq <= '152')
  ORDER BY AccountTransactions.LedgerSeq DESC,
  AccountTransactions.TxnSeq DESC
  LIMIT 400;

became:

SELECT AccountTransactions.LedgerSeq,AccountTransactions.TxnSeq,Status,RawTxn,TxnMeta
  FROM AccountTransactions, Transactions WHERE
  (AccountTransactions.TransID = Transactions.TransID AND
  AccountTransactions.Account = 'rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh' AND
  AccountTransactions.LedgerSeq BETWEEN 415398 AND 415441)
UNION
SELECT AccountTransactions.LedgerSeq,AccountTransactions.TxnSeq,Status,RawTxn,TxnMeta
  FROM AccountTransactions, Transactions WHERE
  (AccountTransactions.TransID = Transactions.TransID AND
  AccountTransactions.Account = 'rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh' AND
  AccountTransactions.LedgerSeq = 415442 AND
  AccountTransactions.TxnSeq <= 152)
  ORDER BY AccountTransactions.LedgerSeq DESC,
  AccountTransactions.TxnSeq DESC
  LIMIT 400;

Type of Change

[X] Bug fix (non-breaking change which fixes an issue)

XRPLF / rippled