Don't scan entire account transaction database when adding transaction.

src/cpp/ripple/DBInit.cpp

@@ -24,6 +24,8 @@ const char *TxnDBInit[] = {
 		Account		CHARACTER(64),				\
 		LedgerSeq	BIGINT UNSIGNED				\
 	);",
+	"CREATE INDEX AcctTxIDIndex ON				\
+		AccountTransactions(TransID);",
 	"CREATE INDEX AcctTxIndex ON				\
 		AccountTransactions(Account, LedgerSeq, TransID);",
 	"CREATE INDEX AcctLgrIndex ON				\