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/test/app/TxQ_test.cpp

@@ -22,7 +22,6 @@
 #include <ripple/app/misc/TxQ.h>
 #include <ripple/app/tx/apply.h>
 #include <ripple/basics/Log.h>
-#include <ripple/basics/mulDiv.h>
 #include <ripple/protocol/ErrorCodes.h>
 #include <ripple/protocol/jss.h>
 #include <ripple/protocol/st.h>