refactor: change the return type of mulDiv to std::optional (#4243)

- Previously, mulDiv had `std::pair<bool, uint64_t>` as the output type.
  - This is an error-prone interface as it is easy to ignore when
    overflow occurs.
- Using a return type of `std::optional` should decrease the likelihood
  of ignoring overflow.
  - It also allows for the use of optional::value_or() as a way to
    explicitly recover from overflow.
- Include limits.h header file preprocessing directive in order to
  satisfy gcc's numeric_limits incomplete_type requirement.

Fix #3495

---------

Co-authored-by: John Freeman <jfreeman08@gmail.com>

src/ripple/app/misc/NetworkOPs.cpp

@@ -2058,7 +2058,7 @@ NetworkOPsImp::pubServer()
                     f.em->openLedgerFeeLevel,
                     f.loadBaseServer,
                     f.em->referenceFeeLevel)
-                    .second);
+                    .value_or(ripple::muldiv_max));
 
             jvObj[jss::load_factor] = trunc32(loadFactor);
             jvObj[jss::load_factor_fee_escalation] =
@@ -2506,7 +2506,7 @@ NetworkOPsImp::getServerInfo(bool human, bool admin, bool counters)
                 escalationMetrics.openLedgerFeeLevel,
                 loadBaseServer,
                 escalationMetrics.referenceFeeLevel)
-                .second;
+                .value_or(ripple::muldiv_max);
 
         auto const loadFactor = std::max(
             safe_cast<std::uint64_t>(loadFactorServer),